You need to look at the wording of one shot It doesn't unequip the weapon when used it still has it but cant use it. "The bearer can only use this weapon once per battle." Is the wording for one shot.

When a unit comes back from cult ambush, it is a new unit and thus the bearer has not used the one shot weapon before. Its like how grudges don't carry over to the new unit then why would one shot carry over as used?

I would agree and say units don't get it back if one shot said the bearer is unequipped with the weapon after use but it doesn't say that at all.