Holy Moly! Based on this post I’ve resolved my problem with the Powerwave 7.5 stand! Prior to these tips - even after stripping my Galaxy S21 Ultra of its case (a Nimbus9 Cirrus2 that contains a magnetic mount plate) - I got a blinking blue light after about 5 seconds of charging. I can NOT believe simply draping an eyeglass cleaning cloth between the stand and phone has resolved my issue and the I can charge in portrait mode again. I’d resorted to landscape placement which allowed continuous charging - explain that!
Further testing revealed however that after reinstalling the case w/ plate (with the cleaning cloth still draped over the stand, the blinking returned, and I had to resort back to landscape charging. Good to know as I figured maybe the S21 Ultra was too tall and didn’t meet close enough to the stand and phone charge plates. Ultimately, the magnetic mount plate seems to be the predominant issue. I just can’t make sense of why the cloth trick fixed vertical charging without a case. Oh well, a fun experiment nonetheless. (Edit: mystery solved. Tried charging again sans case but placed lifted the phone 5mm higher with a plastic object so the camera metal sat above the top of stand. Bam, continuous charging commenced. Fascinating!)
P.S. Yes, I know the stand itself coupled with the supplied PowerPort+1 QuickCharge 3.0 (18W / 3Amp) wall plug is underpowered for the phone model, but it charges sufficiently enough if I’m sitting and working for a couple hours. Of course, there’s always chatter about whether underpowered charging adversely affects the phone battery but that’s a topic for another time and place.