External stylesheet doesn't change layout

I’m not 100% sure but I think this bug exists since the 1.5 update.
I have an external sheet with a media query which I created in version 1.4. Since the update, changes in the style sheet aren’t being applied to the design. Only if I uncheck all single css properties and recheck them in the styles tab, the changes are being applied. If I refresh the page, the changes are reset.
The funny thing is, all css properties I set before the update are sticking and working.