This has happened to me 3 times.

Once with a riflemen, once with a transport and the last time Artillery. All 3 were 3 star units.

When I evacuate a unit the unit shows evacuating but does not evacuate. I can do nothing with the unit.

The opposing team can still attack and kill the unit.

On the Xbox, there was some issue, and I do not know if it was resolved. For a while, it was possible that if a unit was garrisoned and you manually ordered them to evacuate they might have got stuck in the building with the casevac helicopter. I will always exit infantry before evacuating them.

I always exit first then evacuate, but it is annoying.