It may be a combination of sep anxiety and transitions being really hard. At some ages, transition rituals being very short or complex can make a difference. It is just kind of a guess what works for your kid now thing.
Obviously a very different situation, but mine was really responsive to me saying exactly what was coming up for stuff like going to the store, visiting people, etc. I would give step by steps including what was expected of her and what I'd be doing.
Do some searching online for social stories related to your situation. If nothing is ideal, you can get good ideas for DIY. These can help give you all some scripts for the situation.