The behavior stays the same during heat seeking (go towards the heat). The direction might change as it keeps to it's behavior (since the heat moves)

Autonomous means having the ability carry out task and adapt to new information. You are ignoring the second part.

I wouldn't say things like proximity triggers or primitive homing tech like heat seekers are autonomous. Heat seekers don't adapt to changing situations, but follow a completely mechanical "go towards the warmest spot" path. Being autonomous would mean the could react to the "warmest spot" either disappearing or moving in an unphysical way (suddenly appearing somewhere it's shouldn't be, as can happen with e.g. flares).

Basically, if the weapon has a single thing it can do (move towards hot thing), and no way of adapting if that thing doesn't work as expected, I have a hard time calling it autonomous.

I wouldn't call a simple robot-vacuum autonomous either for that matter. If the instruction set is "go forward until you hit an obstacle, then rotate 15 degrees clockwise and repeat", I don't really see that as "adapting to changing circumstances".