There is a limit of 255 siren id’s (carcols) as many are aware (though some aren’t).
Client mods exist to mitigate this. However nothing for FiveM has ever been implemented on the server side.
Is that something that may be adjustable? Pertaining to the issue of 8-bit vs 32-bit integers?
If this fellow was able to do it on the client side, there is some hope here. This limit certainly exists. Though most car devs and server owners are unaware of it. They just randomly type 5435245 or something and hope for the best each time and are using advice from single player mods rather than FiveM.
For the client version that increased siren id’s allowed perhaps you can take a look here if it helps:
(Main Release: https://www.lcpdfr.com/downloads/gta5mods/scripts/28560-sirensetting-limit-adjuster/)
(This is the latest FiveM client sided mod: https://www.lcpdfr.com/downloads/gta5mods/scripts/44272-sirensetting-limit-adjuster-fixed-for-every-fivem-build/)
(Older source code: GitHub - cpast/SirenSetting_Limit_Remover)
Having it built right into FiveM would be incredible. As getting everyone on a server to install client sided mods is not feasible and not a good practice in general anyway.
Sorry that I cannot offer more technical info or reproduction steps about hitting this limit in here. However I believe its fairly common knowledge amongst seasoned server owners or at least some devs I would assume by now.
ps. I have no idea if the fix will be similar to this or not. But its worth noting anyways as mentioned by @Disquse. Workaround for ped props limit in FiveM by Disquse · Pull Request #2421 · citizenfx/fivem · GitHub