Hello,
I found a but in Illustrator CC 2019 and 2020. I will report this to Adobe.
A. I create a glas effect on a non circle object with blending dark to light red and a black and white gradient mixed with ›screen‹ to it.
B. I flatten the transparency with the highest or other option. Always is a clip group (1), that shound be automatical removed. For better seeing, I put it on top of the group ›after flatten tranparency‹.
C. When I put this group in Fontself 3.5, I will see this clip group (1).
It would be great, if there were a filter or option to ignore clip group in fontself 
All files are in the archiv.zip
Archiv.zip (1.9 MB)