Who is your mail provider? Is this account accessed by some other device? How long has this been happening?

We have several email addresses with the same provider. This happened about half a year ago after updating Thunderbird to a newer version. Our company works on IMAP accounts with several e-mail addresses and each account has such subfolders and they are visible. Approximately 12 employees with the thunderbird email client have access to these accounts.

The mail service provider is poczta.vinao.pl and after reporting the problem to them, they see no solution. Additionally, I would like to mention that the problem only occurs with Thunderbird, both on Windows and Mac. After adding an imap account, e.g. to Outlook, everything is ok.

See https://support.mozilla.org/en-US/questions/1371846