Sarado

Basically @PNWParksFan explained it quite well from a coding perspective, but to give a slightly more practical example without technical lingo: imagine being in a pursuit and calling backup. Then you crash into a car that decides to swerve into your path, and you get behind in the pursuit. Your backup doesn't crash into and continues following the suspect. You take a bit of time to reverse, fix your vehicle as your engine is broken and then try to catch back up to the pursuit. Except then, all your backup is deleted by ELS as they passed the (relatively low by default) distance threshold. This results in an LSPDFR crash - and this is what people will see, they will come over to the support forums of LSPDFR to complain about an alleged crash in LSPDFR, while this is caused by improper disposal of ELS.
 
An update to ELS will need to be released to ensure proper cleanup/disposal by ELS, which should be able to be changed pretty easily. Instead of deleting the vehicles, ELS would simply stop controlling AI lights past the distance threshold (I've already let Lt.Caine know).
 
Unused police vehicles will not be all over the map, and ELS's deleting feature was certainly not added for this. It was added so no unnecessary resources are expended by ELS on AI vehicles that the player won't see anyway.
 
The reason unused vehicles (and peds, and other objects) are not all over the map is because of GTA's persistent flag - this is set by either a GTA script or a RPH plugin. Once a vehicle is set to non-persistent, GTA automatically cleans it up. The problem is that ELS does not respect this persistent flag and simply deletes the vehicles. It thus interferes with other scripts' proper operation, even the normal GTA V police response, which can (and has) lead to GTA V crashing entirely.
 
As far as GTA IV goes, that is hardly comparable as it uses a totally different game engine. It was a lot less stable and thus behaviour in ELS IV may have had to be different for optimization for IV - I haven't coded for GTA IV so I don't know what that was like, but it is irrelevant to GTA V and ELS V.
 
The terms 'bug' and 'fix' are used here to accommodate for those who may not be such technical experts. More accurate terms could have perhaps been used, as the deleting behaviour was intended in ELS - I didn't notice the effects while testing ELS as I hadn't filled my slots with ELS vehicles. 
 
Setting the max distance to a higher value is only a temporary and certainly no full fix, as it can not be determined how far away script-used vehicles are spawned from the player. It merely mitigates the issue and makes it less common.
 
Hope this explains it a bit better. It's certainly not an unfixable major disaster and in no way degrades the usefulness and greatness of ELS. It's just a pity it slipped through while I was initially testing it. Testing role never ends, I guess 
Cheers.

It's absolutely a bug, and just because ELS IV may have had a similar bug doesn't make it not a bug. 
 
The RAGE engine lets you mark an entity as persistent. That indicates to the game that it should not delete that entity. ELS isn't respecting the persistent flag. Therefore, that is a bug. You can argue about how plugin developers "should" write our plugins, but there are plenty of valid reasons why one might want to spawn a vehicle far from the player, and it shouldn't be up to the ELS developers to decide when we can and cannot spawn vehicles. For example, perhaps you might have a pursuit of a stolen police car... well if you're not doing a good job keeping up, that police car could be further away than the ELS limit and get deleted. 
 
You also say that it's easy for plugin devs to simulate with a time delay, but there's no good way to get a point on a route from point A to point B, so if you want to realistically have vehicles approach on the correct route they'd take from one point to another, I'm not aware of any solutions. If you can think of one, please let us know as that would be very useful for a number of things.
 
While it can be fixed by changing the number in the INI, the point remains that the default behavior introduces unexpected issues with quite a few plugins used by many users. The average user can't be expected to a) realize the bugs they're encountering are being caused by ELS, b) locate this forum thread, and c) change their configuration to make their plugins work again. ELS needs to be updated to respect the persistent flag, plain and simple. 

Edit: This has now been patched. Please download the latest version.
 
To those crashing on the Assorted Callouts Traffic Stop Backup Required/Stolen Police Vehicle calls, or those having issues with a plugin that spawns emergency vehicles at some distance away (e.g. BetterEMS by @PNWParksFan ):
 
This is caused by ELS taking over the ELS vehicles these plugins spawn if you have a VCF installed for them.
Because they spawn a distance away, ELS cleans them up instantly and deletes them.

Thus, an ELS update will have to be released to fix this.
For now, you can change the ElsMaxRangeAi setting in the ELS.ini file to increase the range before ELS vehicle are cleaned up. Change it from 500 to something like 2000. Do note that the higher this number, the more significant the performance impact may be.
Alternatively, you can disable ELS taking over AI in the ELS.ini file, or remove all ELS vehicles from the default emergency vehicle slots (ELS addon vehicles are not as likely to cause this, as most plugins only spawn default emergency vehicles).
 
I have been getting very many questions and reports about issues caused by this, so I figured I'd post it here. Feel free to link those that apparently don't know to this topic.
 
Full technical explanation for those who are interested, click here.
Cheers.