I have the same again now with iOS 12.1 (few weeks went without the bug on iOS 12.1) on iPhone XS Max 512GB Gold.
So the bug is still there!
Killig all open apps did not do a thing. Reboot fixed the problem.
I reported the bug again, this time by calling to customer support, they ran diagnostics on the iPhone via remote connection and everything was good, no problems on hardware etc. The customer support told that there is no solution now but the bug is now reported and and the engineers will look in to it.