The monitors are: BENQ, 32", 3840x2160, set to text size 150% as recommended by Windows; ASUS 27", 2560x1440, set to 100% text size.
1. BENQ as main display.
1.1. iMatch file window on ASUS, panel with Favorites on BENQ. For the test, I right-clicked on an item in the Favorites field and looked at the PUB (pop-up box). 
1.1.1. The PUB appeared adjacent to the cursor, but with small font size (I would guess about the 100% setting for the BENQ monitor). I also tested the dictionary PUB in an Attributes field, with that panel on the BENQ also, and found the same result.
1.1.2. Favorites and Attributes now on ASUS monitor, with File Window. Both PUBs appeared normal (Image 1). From here on, I mostly used just the Favorites panel.
1.2.  iMatch File window on main BENQ monitor
1.2.1. Favorites on ASUS monitor. Shadow of PUB appeared in the correct place (see image 2), but text box was significantly displaced to the left of its correct position. If the Favorites panel was set close to the left edge of the monitor, the PUB would not be visible. Text in the PUB was normal size. I tested with the Attributes panel and the dictionary PUB was displaced left and down of its correct position (Image 3).
1.2.2. Favorites on BENQ monitor with File window. PUBs worked as expected.
2. ASUS as main display. (I closed and re-opened iMatch at this point).
2.1. iMatch File window on BENQ monitor. 
2.1.1. Favorites on ASUS. PUB shadow in correct place, text displaced.
2.1.2. Favorites on BENQ with File window. PUB works as expected.
2.2. iMatch File window on ASUS monitor.
2.2.1. Favorites panel on BENQ monitor. Shadow of PUB in correct place but text box slightly displaced with small font (Image 4 ).
2.2.2. Favorites on ASUS monitor with File window. PUB works as expected.
3. I haven't tried changing the text size settings as the 150%/100% described earlier gives fonts of the same visual appearance on both monitors.
4. Closing and opening the panels does not change the results.