Opera's requirements page is usually up to date, but there have been a few times where the macOS requirements on the page were incorrect. Double-checking with https://support.google.com/chrome/a/answer/7100626?hl=en is a good idea if you have problems.
This is still happening, I move a tab to another monitor, and even dragging it to the top of the screen, it does not maximize, it seems like it is but its not. It's absolutely annoying and needs a fix!