I think there's simply a bug in the radio interface layers. I sometimes have system-ui show that I don't have coverage, but this is cleared up by entering and exiting flight mode.

By the way, if you are on a carrier-variant version of PR1.3, you cannot use a generic variant of PR1.3 (save for a creative replacement of the rootfs, skipping Flasher entirely) (this will not replace your radio firmware)