Hello,
I found a but in Illustrator CC 2019 and 2020 MacOS 10.14.6 and Windows 10 v1903. 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‹.