Great!
There has been a mysterious “…some sequence of user actions…” that has been causing these tiny artifacts.
But it seems implausible that suddenly this month many Shotcut users all simultaneously began micro-adjusting tracks, which no one had ever done before.
It looks more and more like something changed several weeks ago, perhaps on or about the 29th of January, that is causing Shotcut to handle these micro-adjustments differently.
And now, thanks to the careful observation and reporting by @alanclarkeau, we may have a way to reproduce it.