Not sure if this is a bug. When I update a theme I would the next time, even during a live service, expect the udated theme to be used. This only happens when I shutdown openlp and start it again.
Which version of OpenLP are you using?
OpenLP 1.9.5 build bzr1641
I don't have to restart it, I do have to switch the theme then switch back, and that seems to solve it for me.