I was able to consistently reproduce this bug pressing J in the right time I got this error:
panic: runtime error: index out of range  with length 19 goutine 117 [running]: git.sr.ht/~sircmpwn/aerc/worker/imap.(*IMAPWorker).handleFetchMessages.func1(0xc0000b0e) /src/worker/imap/fetch.go:80 created by git.sr.ht/~sircmpwn/aerc/worker/imap.(*IMAPWorker).handleFetchMessages src/worker/imap/fetch.go:78
If I change the folder from "Invoices" which has 19 messages, to "Invoices/Amazon" which has 44 messages, then aerc panic with the above message letting the terminal unusable.
The movement (using J) needs to be done before finishing rendering the messages in the Folder A, and then move to a non-empty Folder B.
Using aerc 0.3.0