Yes, fortunately a fix for this exists. Pardon the bump, but I feel like this is necessary.
Undoubtedly, individuals who have custom car add-ons within their own FiveM servers are potentially under subjection to this particular issue. This does not necessarily mean, however, that the exact same add-ons that are problematic for some, are also problematic for others (such as in my case).
To date, I’ve seen several other threads that speculate that—
• a 1992 Dodge Ram add-on is responsible for it,
• that the (add-on) trailers’ weights may be modified to correct it,
• that it is also unfixable, and/or caused by other means,
• and threads whose participants have either never provided any actual information about their solutions or were left solutionless, but fortunately some of the aforementioned circumstances were never actually the case or cause of affliction for the trailers that were affected within my own FiveM server.
A user named @Felix_Fox who posted in this thread about the same issue managed to come close to proposing its resolution; by not only denoting one of the exact add-ons that were causing the bug,¹ but proceeding to state that the vehicle’s handling.meta file was probably responsible for reproducing this issue.
In my case, however, I had two particular vehicle add-ons (the 2020 Ford F350 King Ranch and its dually variant, namely) which possessed nested vehicle and handling line configurations for other add-ons (by the same author) inside of their own respective vehicles.meta and handling.meta² files. Why? Probably for the author’s convenience.
For what it’s worth, I actually wanted to keep these add-ons in my server, so I did not want to simply delete them; I’m in the process of creating a vehicle add-on pack for a community server that I am a member of, and the owner of that server has requested several times that I install this pack on his server. One or more of the nested vehicle and handling line configurations (for the other add-ons) within these add-ons’ own respective vehicles.meta and handling.meta files have presumably contained configurations for a trailer add-on.
I’m not certain about which of the nested vehicle and handling line configurations were solely responsible for producing the trailer bug, but I am willing to believe that one or more of them were either misconfigured (since trailer and vehicle configurations probably shouldn’t be nested together at all), or nested erroneously enough to somehow break all of the trailers, yet the latter didn’t actually seem to be the case.
The solution to this issue, in my case, was to simply remove each of the nested vehicle and handling line configurations from the add-ons’ respective vehicles.meta and handling.meta files, in order to remove the trailer add-on associations of the configurations that should have never been nested within these files. A very simple process.
I assume that the author nested these files’ add-on configurations inside of these add-ons’ vehicles.meta and handling.meta files to retain their consistencies when used in singleplayer. At the same time, I assume that he also never really intended for these add-ons to be used by anyone in FiveM, hence the manifestation of the bug itself. I didn’t have any other active resources other than vehicle add-ons on my current FiveM server at the time this issue manifested, so I had no choice but to investigate them in order to reveal which one was the cause of origin; now we’re here.
¹ This URL is referenced as a location for the add-on in context because the original one was removed from GTA5Mods’ website. Moderators are welcome to remove it at their discretion.
² These URLs contain raw vehicles.meta and handling.meta file data for the 2020 Ford F350 King Ranch add-on; cited contextually for those who want to see the nested add-on configurations within them, and/or may be experiencing this issue in sought of a resolution for themselves.