I did a bit more investigation, and found that "-f" outputs huge amounts of debugging information. In that I was able to see that after 2 movement commands it is detecting a file changed event, and the "newFiletype" is "plaintext" where it was previously "python".

It doesn't seem to happen if I rename the file I'm editing to have a ".py" suffix. "foo" suffers this fate, "foo.py" does not. Submitted to Issue tracker, didn't look like it was already in there.