Doug,
You're correct about the barrel lock-down on the Ithaca.
However, here's a story that might mitigate the swirling gasses aspect:
George Rhoads designed a sculpture called the
Windamajig. Several examples were fabricated, but an occurrence with the first one, basically the prototype, might shed light on this topic. I wrote most of a description of what it does, but will save the read and direct you to this video of it in action:
https://www.youtube.com/watch?v=kok97EfvJbk
The example in this video has mounting plates, but the prototype had a threaded pipe coupling screwed to the main post. No one took particular notice that the thing turned counter-clockwise, that is, until the entire sculpture fell off the post--having unscrewed itself--
with people watching!
If swirling gasses can un-screw a suppressor, why not use a left hand thread, or a gun with a barrel rifled the other way?
I think heat and pounding set the stage for looseness. With the '37, it was advised that the keeper ought not to be screwed down very tightly if the barrel was hot, lest one find oneself shooting back over one's shoulder!