Monitor resolution keeps resetting

[Rich Shepard]

I've replaced an HP/Compaq LA1951g monitor (1280x1024) and a Radeon RX-550
video card with an Acer SB-220Q monitor (1920x1080) and a Radeon Pro WX-2100
video card.

The monitor will accept a refresh rate up to 75Hz; the WX-2100 supports a
60Hz refresh rate.

Using Settings -> Dislay -> Resolution I set it to 1920x1080. If I'm away
from the desktop for a while, or log in, the resolution has switched back to
1280x1024.

Why might this happen and how can I fix the 1920x1080 resolution so it stays
there? (I've been using xfce since it appeared and haven't had this issue
with any previous video card and monitor over that period.)

Regards,

Rich