I have just installed 9.10.0.21 and the fix for "Fixed an issue where ARC would show up off the visible desktop if previously opened on a screen that was no longer attached." now opens ARC on monitor number 1 every time.
It used to open it on whichever monitor I last used. I realise this is an easy way to fix a problem if you close it on monitor number 2 then try to open it with if that monitor is no longer attached, but it's a pain.
Surely the number of instances of people opening ARC with a different screen configuration is much smaller than the number of instances being the same as last time, so more people will be affected by this change than it will have solved a problem for.