Out of phase signal shifts radically

I feel for you as I have chased the exact same issue with a spdif cable I had ( past tense most definitely!).

This was one that was of the locking rca end type and I guess it was already in its locked position when I pushed it home and like yourself I thought it was way tight and not possibly the cause of any issue, yeah right!

After wasting a couple hours wondering what the heck was wrong I tried different cable and perfect! Only then did I examine the culprit cable carefully and see it was a locking type that was already "locked" so preventing it engaging fully!

And the solution...

Sound got better and had not been a problem until a few days ago. To me it sounded like a mono signal with one channel out of phase (not possible, I know). Test bass would not even align left and right. The left speaker had the bass 2' to the right. The right speaker had the bass center stage.  Out of phase went way LEFT - instead of right as above - and pinpointed just behind the speaker.

I've been shuffling components and have several coaxial cables. One of them is really tight and difficult to force on - nearly impossible. Vision is poor and space is tight as I'm working between the wall and back of stereo (I have more excuses if I need them...;)  The cable was forced on hard, but still only about half way on. Really pushed to get it in a bit further and PRESTO - everything came into alignment and I had great music again.

In all my audiophile years never had this happen - and it is so obvious. Perhaps more interesting was my brain trying to justify this terrible and almost mono sound coming out of the speakers. I thought it wasn't correct, but questioned my hearing, the room, switched amps, etc. and almost excepted it as just bad sounding speakers - even though I was quite happy with them before.

Bizarre. I've observed some pretty dramatic effects from room acoustics / changes, but this seems to be an exact match with the out of phase signal. The strangest part, to me, is that normal stereo play - though not perfect - sounds pretty good.

We ordered new blinds and they should be here in two weeks. I'll report back unless I can't wait and cover the glass with some sound absorbing and/or diffracting agent.

You have probably nailed it. The first question when solving a problem that just cropped up in any kind of previously working system is “what changed”.  The sanity check is to put each item that was removed for the renovations back, one at a time, and re-run the test. The reason your out of phase output sound has “disappeared” in the left and center is due to cancellation from reflections. Your room is apparently an integer multiple of the test tone wavelength. Cool eh?