Multiple power cycles later, MC and my Marantz AVR + LG OLED are playing well together again. WASAPI exclusive with HDMI bitstreaming within MC, as before.
Origin of bizarre issue unknown; however, I did add another component to my Audio/Video system between proper MC behavior and anomalous behavior... I got an OPPO UDP-203. It uses a different HDMI input on the Marantz AVR, of course. Although it makes absolutely no sense to me, it appears possible that the Marantz does not "like" shifting from the HTPC (running MC) to the OPPO, or vice-versa, unless I give it a power cycle (even though playback was stopped on either device). It will change inputs as directed, but with the quirky audio behavior.
If these observations hold consistently, then I will have a simple work-around: power cycle the Marantz after switching among its HDMI inputs.
