Vesion: Geany 1.38. OS: Fedora 37. Graphical Environment: Mutter (Wayland).
I seem to have a similar issue. In my case, when the secondary monitor is connected, the pop-up autocomplete dialog is only visible on the monitor configured to be on the left or top.
Changing the primary display in fedora settings changes nothing.
Sometimes i can see the pop-up in the secondary monitor at right (which is bigger than the primary), but it has a large offset to the right. Seem to be an positioning error rather than the pop-up being invisible.
I hope that this information will help find the problem.