It's also possible that they were just trying to protect a student from investing years into a project that they deemed to have a low (but perhaps non-zero) cha

It's also possible that they were just trying to protect a student from investing years into a project that they deemed to have a low (but perhaps non-zero) chance of success. This is a thing that good advisors should do to protect their students from career-wasting wild goose chases. The fact that the two interactions were very different with four years and a completed thesis between them doesn't surprise me at all. My own embarrassing story is that I advised Jason Donenfeld to submit his WireGuard paper to NDSS, forgot about the meeting entirely after a few months, then complained (in retrospect, unfairly) when NDSS accepted it. Advisors do stupid, embarrassing, forgetful things all the time. The OP's story isn't even a misdemeanor.