I'll find out in few more weeks maybe. But it makes sense. Sleeping on the side of your body most affected by TOS can prevent healing.
I've been a right side sleeper my whole life which is the side most affected by TOS.
Before I started correcting my posture in my mid forties (I'm now 71), I'd experience weakness and numbness in both hands. Both arms would get fatigued almost immediately if I tried to exercise them. I became interested in juggling and it took me a long time to learn basic juggling because my arms would get super fatigued almost immediately. Raising my arms over my head made them super tired another sign of TOS.
Since correcting my posture and using a back brace and stretching the only weakness I experience now is on my right arm. So at least no problems with my left arm and hand anymore.
But not sleeping on the side of your body most affected by TOS makes a lot of sense. I probably minimized the good of stretching, good posture, and the back brace by sleeping on the damaged nerve bundles for 7-8 hours every day.
We'll see what happens.. I would like to hear stories from people who improved or cured their TOS situation by changing their sleeping positions. I could not find any stories about that possibility on the internet. But I did see a number of admonitions about not sleeping on your TOS-affected side of the body. I hope that was the problem.