I have the same problem where the alt windows just disappear on 'monitor 2'.
I can run the main window + 1 alt and that one opens as you would expect on the 2nd monitor without issue; however, when I try to go with 2 or more alts, the windows start loading on the 1st monitor, will flash/load for a split second on the 2nd monitor, then disappear altogether. Frustrating!
If it's a scaling issue with the Alt window size calculations, which entries would I need to 'play with' to possibly correct this?