Did you create that top folder by using an '.' or '/' character in the name?
Then this is normal behaviour, because on creating that folder your IMAP server doesn't set ACLs for you on it.
Only the sub folder gets the needed privileges.

The only way to change that is to connect to the IMAP server with an admin account and set the desired ACLs by hand.
With cyrus you would use the cyrus user for that.
Afterwards you can delete it.

No, I add a folder in SOGo -> foo12
In there a subfolder -> bar12

So I have foo12 -> bar12

If I delete foo12 the name is italic, and bar12 is normal and both folders are in the folder hirachy and not deleted.

From TB 31.2 this works. Deleting foo12 delete foo12 and the subfolder bar12

Sorry, I can not reproduce that behaviour.
When I delete a normal folder with subfolders, then they are all moved to the trash can.
After emptying the trash, it is gone.

Only if I don't have privileges to delete/modify/use that folder, then it will be grey.

Which IMAP server are you using?

hm strange, I tried two browsers, different time. Deleting fails.

We use dovecot 2.2.13

This is true for dovecot and mdbox storage.
The top folder gets "deleted" and the sub folder stays as is.
The grey color is indication for no access rights to this folder.