Sadly, yes. A Proton team member on Reddit confirmed it a couple years back:

Folder/label names are visible to the server (for filters and other reasons) as are email metadata. Message and attachment contents are encrypted and not visible to the server.

https://old.reddit.com/r/ProtonMail/comments/hxomoy/are_mail_folderstags_encrypted_or_private/fzc6wry/

Do note that this is ONLY true for Mail. Calendar, and Drive does encrypt folders/label names.