1.2.2, 1.2.1 and 1.2.0 are the first software releases using QT5.4.1. Previous releases used QT4.8.1
My guess is that's related. Someone reported what I suspect is the same issue with 1.2.1, also on Xmonad. They said that the popups in our software closed as soon as they opened. (most of our menus are actually custom popup windows).
We loaded Xmonad on Unbuntu 14.04 in a VM when the first issue was reported but we weren't able to reproduce the issue. However, we did notice odd transparency issues with the popups. (black backgrounds where the alpha channel should be, and colored boarders)
Did 1.1.34 work? Which distro are you using? We'll need to reproduce this over here in order to get to the bottom of it.