I had the same problem.
Double-clicking the main display area is supposed to immediately launch "Display View", aka full screen mode, but in MC28, it no longer worked for any 3rd party plug-ins. If you switch to the Cover Art display option, double clicking that will still launch full screen mode. It is just 3rd party stuff that did not work. FYI... you can right-click and select "Display View" from the context menu to still launch it... it is just the double-click that is ignored.
That said, it suddenly started working again (for me) on 3rd party app's. Not sure which version it came back on... I thought it was v98, but I could be wrong (since that is what you are on and it is not working for you). It's definitely working right now for me on v100. Not sure why it stopped/started working to begin with, but I too, was none to happy when it stopped working.
Hopefully updating to v100 will fix it for you. If it does not, then maybe there is setting difference between our set-ups that is causing the issue? though I have no idea what it would be... and I pretty familiar with the settings.