Mail folder icons

Hi. I recently set up a Nextcloud instance for a small volunteer group (local theater). They use another mail provider than my usual one, so I configured the mail app to use that provider. Subsequently I pointed the mail app to the Send, drafts and Trash folder in Account setting → Default folders.
But on this instance the folders icons is not replaced to represent the assigned function. Is that something setting missing on the mail provider side (I also had to create the folders myself) or can it be due to that the folders on the new site is subfolders to the inbox.

My personal mail on the left and the new instance on the right. Kladder is Drafts, and Skrald is Trash

Any guesses on why?