This isn't a "bug".... This is how Illustrator functions and has always functioned where Opacity Masks are concerned. A "bug" is an instance of repeatable behavior which is unexpected. What you have outlined is entirely expected behavior.
Opacity Masks assume the top object of a selection should define the mask. It doesn't see "groups" So the top single ...
I think you are trying to move some color fill layers from an RGB document to an Indexed document.
Go to the animation file and from the menu Image → Mode, check if Indexed is selected. If yes, change it to RGB.