Some HUDs are non-exist, I believe the Tri and ARC suffer from this
But make sure to differentiate between what is a regular vehicle and a dev vehicle depending on layer. Dev vehicles are WIP while the regular vehicles like ARC and Tri suffer some issues that are being fixed with these new ones.
Somes HUDs as well just disappear due to a base Squad issue of switching seats from one HUD to another (I.E LAAT gunners switching from one gun to the other)