I have the smilar issue running Wyse 3040 with ThinOS 2205. I had the issue in earlier versions as well. I have specified the resolution to be 1920 x 1080 on both screens. The issue occurs for us when the unit is woken from having been idle for a while and the screens are in sleep mode.
I have been restarting the entire client when the issue occurs, but thanks to your tip I now only have to turn the display off an on again. However, I would like to have this issue fixed. After over a year of testing ThinOS 9 and fixing out the quirks we finally deployed in production last week and this issue pops up.