I have noticed this problem with aMule-rc4 and newer, I don't know if it was present also in older releases.
In search tab when I click on Category dropdown menuI get a normal dropdown menu:
But after 2 or 3 seconds it disappears and leaves only a grey square:
No problems with type dropdown menu. This happens with both wx 2.4.2 and 2.5.1 linked to gtk1. I have not tryed gtk2.