"Has anybody ever found a spot in Oz (apart from the various CBDs, deep canyons, tunnels, very dense jungle, or underwater!), where GPS won't work

"
I've seen it happen in areas in Central Australia and a few times on the Canning Stock Route, where the gps will stop responding and not update the screen and then some time later (no specific time period) will just as suddenly start behaving normally again.
This was back before Covid, so I don't know if there was a small gap with no overlapping satellites, needless to say there was no mobile service from Wiluna to Halls Creek