I wrote a very detailed, step-by-step testing response but did not save it externally before clicking the Dropbox link in previous message to confirm it still works. That clobbered the message. Will try to recreate from memory.
Bottom line is that v 4.11 and v 4.22 have the same behaviour with the corrupted textboxes. However, v 4.22 did work properly on occasion by swapping between versions, but I could not find a reliable way to make the text boxes work properly. On the other hand, playing with the Scrolling / Non-scrolling setting in the textbox definition did seem to always cause the diagonal scrolling corruption.
One textbox contains three lines of white, centred text. When it is working properly, it simply scrolls down the screen. However, when it is corrupted, it acts almost like a Photoshop layer mask in that there are both white and black copies of the text. The white text scrolls vertically, but is hidden except for when it is superimposed over/under the black text which scrolls on a diagonal line. The same behaviour is present in single-line textboxes, although it is harder to see because the text occupies less screen real estate.
The dropbox (see earlier message) contains two project files and the video diagnostic results. The original one is the original project. The V-3 copy is from when I found the textboxes to be working properly and SaveAs immediately into a new file. Sometimes the V-3 works properly, sometimes it has corrupted textboxes. Could not find a pattern.
Don't know if this stuff is useful, or just a red herring. Hope you can find the cause.