I have some groups with icons as symbols on DT:
In DTTG (iPad), I get a black folder symbol fro them:
Not a big deal for me, just to let you know.
These custom icons were PNGs, SVG, …?
I don’t remember 
I went to some website and converted emojis to whatever format they offered. Probably PNGs.
Have you checked View Options (three bars) > View… > Thumbnails?
I hadn’t, but have now and everything is as it should be. Thanks a lot!