OK, I could reproduce the bug.
It looks like some kind of messaging race condition between the viewer and server (thus why changing another parameter, such as Phantom or Temporary, "resyncs" the faces data). Note also that if you log off and back in, the prim looks normal (so the wrong face repeats were not actually sent to the server, at least not until you change something else on the face).
So far, I could not find the reason for this bug (and it may take me quite some time to find it). "Planar" mapping has always been buggy and a PITA, and does not properly work either with GLTF materials (LL's problem, not mine)...