DB out of sync secondary node HAG
Imho, it's worth to try the following:
Simulate the same scenario but with a tiny test db. Don't forget to do full/log backups which happened (?) with your big db on primary since Sunday.
It will take ~10-15 mins to reproduce and you will know for sure will it go or not.
Regarding the question:  I believe it will not start auto-seeding because your DB was recovered and readable before you removed it from HAG. Also, if there was a full backup on primary, it also affects the log chain.