I'm using Minecraft 1.16.5, vehicle-mod-0.45.2-1.16.3.jar, on forge 36.1.18.

When a trailer is detached from a moving vehicle it will keep moving indefinitely, without slowing down, until it hits an obstacle that it cannot climb, such as a wall. This also happens when moving the trailer by hands if released while moving. I tested this mainly with a chest trailer.

Vehicles do slow down if you jump off, so this seems to effect trailers only.

Also: I realized this is just a duplicate of issue #450 but there was no description or title explaining the actual issue. But the video included there shows the issue.

And a duplicate of issue #448 . Sorry