This only happens on mobile devices. To reproduce: open Thread Display Options anywhere on forum. Notice how the element that is autofocused looks like a simple textfield. Click on another select element and it also now misses its usual appearance, but the previous element looks fine.
Tested on iPhone 5, iOS 6.1.4.
This seems to happen when a background (image) is applied to the select. I don't understand why that effectively turns the appearance to "none" but it's easy enough to fix. It only needs to be applied to select.textCtrl.