There is an issue with editing opacity masks in the latest version of Illustrator 2021. There is a serious lag and the effects of editing only show up when you click out of the opacity mask editor and go back into your normal mode of working. It's really annoying and causes many errors and time wasting.
In my experience, there have been problems editing Opacity Masks since, well, forever. So much so, I do not use Opacity Masks, ever!
As Ton suggests, vote so the issues get fixed.
Are you serious ? Yhis one of the function I certainly use the most in Illustrator ? Since when did you use Illustrator ?
Try editing your Opacity Mask. Do it a couple of times and it will could stop working.
By the way, I've been using Illustrator since 1988.
Also been an Adobe Certified Expert in Illustrator since since Illustrator 9 (two versions before Creative Suite).
Once upon a time, opacity masks used to be the *only* reliable way to colourise greyscale (raster) images with spot colours in Illustrator.
So, at least now we know that Jeff never used that capability 20 years ago. 🙂