If you nailed it, not too bad to fix, either!
The "fix" is/will be the conclusion of a very interesting/frustrating scenario.
That washer wall mount valve is OLD, and dripped for a while each year, when I turn the system back on after it having been asleep (drained) for Winter (Covid changed this Winter for me).
A few weeks ago I decided it was time to change that valve (the new valve had been sitting in a box for 2 years), and finally solve the dripping issue.
Changing the valve was going to be a project, involving moving stuff, removing the old valve, and replacing with new, all in a rather difficult spot.
Ah Ha, I thought, these valves appear to be identical.
I'll just take the guts out of this new one, and put them in the old one.
These are both high quality brass valves.
The drip was fixed, and all was well......until something went wrong....like NOW!
I did save the new valve body, so just in case, I could put the parts back in, and replace the entire old valve assy. with that.
Ain't gonna happen!
I am 80 years old......gonna buy a complete new valve assembly tomorrow.
When am I gonna get the urge to put it in?
I dunno!
Hot water works normally..... while the valve is off,...... and it only needs to be on for laundry!
2 more years maybe?