There seems to be a bug with system text given a custom scale. The linked video shows that when you edit the system text, it keeps moving up the window. In galley mode, it eventually disappears off the top.
Yes, it does seem similar. However, I don’t think it is due to collision avoidance. It just gets into a mode where the text keeps going up and up and eventually out of display range, even when there is nothing to avoid colliding with. I had a rehearsal mark, but when I dragged it to a different measure, my text dropped back to the expected place – but then started climbing again with every edit.
I could only produce this behavior on system text where I had set a custom scaling. But the architecture of Dorico seems to make it very un-deterministic – very hard to reproduce behaviors. So the problem may have nothing to do with custom scaling.
P.S. I do search the forums before reporting these things. But the bugs tend to show up different ways for different people, so we tend to describe them differently using altogether different keywords.
No worries. Just a possible connection. I don’t do the search slap. 
Nor was I suggesting that. I appreciate your adding that link. That makes these threads a more valuable resource for those trying to de-mystify the world of Dorico. It has already become a very sophisticated program with a great deal of complexity. If the program is well adopted (as it seems to be,) it will be impossible for the Steinberg staff to help everyone get up the learning curve. And the more self-help that is available via these threads, the more resource Steinberg will have to advance the product development.