This follows the mailing list "discussion" https://lists.sr.ht/~rjarry/aerc-discuss/%3CCSLCE7V6PBJA.THVFF075ARCT%40walnut%3E#%3CCUCSBPWQPMTR.1YRS06PUGJK6G@walnut%3E.

When using a maildir account, at startup, unread "current" messages in subfolders (in maildir terms, the ones within the "cur" directories that contains mails "seen" by aerc before) are displayed correctly. However completely new (so also unread) messages in subfolders (in maildir terms, the ones within the "new" directories) are not displayed.