I found a solution for my Tapo cameras. The short answer: I set the RTSP transport to UDP instead of TCP. With that, the motion artifacts went away.
More detail: This was not necessary with the SimiCam – only with the Tapos (I have three different kinds: C210, C310, C320, and they all need it).
I found this using ffmpeg as I showed above by changing the -rtsp_transport from tcp to udp. This made the non-monotonic warnings go away.
Also, as a test to ensure that wifi really was not the problem (I has pretty proven it wasn’t earlier), I connected via Ethernet (C310) and got the same behavior.