Same issue different manifestation.
The rules do not tell us what to do. Discuss with your opponent/gaming group on how you would like to proceed.
Your options are essentially:
A) embarked models are immune to morale/Ld tests.
B) embarked models take morale/Ld tests normally, but if they fail you need to create house rules to allow them to fall back out of their vehicle.
Neither solution is supported by the rules but both solve the problem nicely depending on how you think the game should be played.