As posted on the Cockos forum (here’s the video)

GREETINGS. I’ve been using Reaper for more than ten years for its wonderful reliability and had experienced zero problems until recently. I posted about a problem initially on the Reddit sub but wasn’t able to solve it even after verifying all settings and even deleting the Reaper.avi file as a last resort a responder recommended.
Here’s what’s going on:

Suddenly, monitoring has become strange, L channel is being sent to both L & R monitor outputs and the R channel does not output at all Thus all I hear is the channel in L-channel in mono out of both hardware outs. Panning the Master to the right produces no output at all.
All plug-ins are disabled. Hardware routing has been verified as correct. This problem exists regardless of which hardware I use. Audio interface, internal speakers, external speakers, bluetooth headphones = no difference.
Important to note: this is in monitoring ONLY. the meters show stereo and bouncing Master output to a new file produces correct stereo. So it seems only a mismatch in monitoring somehow. But it makes it impossible to work.
Also notable: This problem exists on any file, new or old, created before or after this bug was noticed.
here’s a link to a short video where you can observe this behavior.
MacBookPro w/ OS 15.3.1, Reaper 7.33 64-bit (the problem occurred before updating Reaper from 7.3) 64G RAM
Any advice on how to solve thios would be greatly appreciated.