Using the following as the fvwm configuration file will cause the GeometryWindow to have the size of a single pixel.

DestroyModuleConfig TestButton: *
*TestButton: (1x1, Title "TestTitle")

Module FvwmButtons TestButton
Wait TestButton
GeometryWindow position 50 50

If you move a window/resize a window you will notice a single pixel appear in the middle of the screen, which is the GeometryWindow. Setting the DefaultFont after this will fix the issue.

DefaultFont "xft:Sans:Bold:size=8:antialias=True"

Using only the single line Wait BadName in the configuration file will cause the GeometryWindow to not appear (it isn't even a single pixel and changing the DefaultFont as above won't fix it). Might be an issue due to being set in the Wait state. Hitting Ctrl-Alt-Escape to abort the Wait command doesn't help either.

In both cases this only happens when used inside a configuration file that is read when fvwm starts. If used after fvwm is loaded in FvwmConsole or Reading a configuration file, things work normally.