I just tested it a bit, and it seems like I can't reproduce the powerplant Ctrl+Shift build cancels mex issue unless Units/Waypoint Dragger (Widget List Only AFAICT) is on, since the cursor is still over the mex when doing that, and when mexes are dragged using Waypoint Dragger they are treated as placed on invalid terrain and cancelled. Also the powerplants aren't queued. If Waypoint Dragger is off, then there's no issue with powerplants.
Also, it looks like the issue where shift+areamex toggles whether a mex is built is still there, and I've no idea why. I guess it's a consequence of other code interacting weirdly, but areamex shouldn't cancel mexes. It makes quick mex planning far more finicky than it has any right to be.