Problem with group by folder.

[Alime]

Hi and sorry for My bad English.
I want to musicbee show my folders in artwork view mode and then sort it by date modified (Desc).
Also there are some files in library with empty tag.
This is What I do:
1. preferences > sorting/grouping > check files for each album are organized ...
2. preferences > sorting/grouping > make a custom sort > date modified (Desc)
3. set library view to artwork and group by album.
The problem is when I sort them by custom sort(date modified) some of those files without tag appear in list. and they have "unknown album" title.