I was having the same problem on my PC with multiple monitors. This thread helped me work around the problem:
GeoNet - Toolbars won't stay where I move them(!)
Here is the specific post from the above thread :
So what I did was worked with one toolbar at a time.
My procedure for fixing my toolbars:
1) Click Restore Down to see what toolbars move.
2) Click Maximize 3) Undock troubled toolbar and leave it floating
4) Click Restore Down, does it move? If not, place it where you want
it. If it does, you still have a problem and this workaround will not
5) Click Maximize and Restore Down to test if toolbar moves (this is
where it stays put! awesome!)
6) Repeat steps 1-5 for all other toolbars, but I never tried to do
them all at once.Here is a screenshot of the settings needed in
Toolbars > Customize > Options tab
In my case, it was the "Advanced Editing" toolbar that was causing problems. Now I add it when I need it and then remove it as soon as I no longer need it.