When games update often it is always a good idea to load in with no trainer, open a few menus, do a few things and re-save in case there are changes to the values the trainer looks for and you save gets updated with those values untouched. Try that or maybe a new game, if it activates and works on a new one try exiting loading the save and try it again. Recommend turning off cloud saves as they can be uploaded in an altered state,
Started a new save (Freedom - no hunger/thirst - Mode) and played the game legitimately until I was able to unlock the Seatruck. Built it, saved, activated the trainer and only the Infinite Vehicle Health option: the Seatruck still took damage from both collisions and enemies.
Closed out the game and the trainer, started the game with my existing Survival mode game, played it for 15 minutes (I timed it) legitimately doing various things (even used the Seatruck to make sure it took damage normally), repaired the Seatruck to full health, then saved. Opened the trainer, activated it and then only the Infinite Vehicle Health option: the Seatruck still took damage from both collisions and enemies.