Automatically moving the VM would have the same problems, no human intervention. Calling a script (for example) which does that would trigger the same behaviour as when we would implement it in the HA manager, I don't see why one should be safer than the other. But I could see that for some admins it would be more comfortable.
The logic is that a reboot is a planned action and we do not want trigger automatic things on such an action (a reason you mentioned also), also a possible out of control feed back loop should be avoided.
Manually unfreezing a service (e.g.: to a machine) should be thought about, but it's not that simple.
(Not the nicest and "no warranty") Work around for you, kill the pve-ha-lrm process and then reboot, the services will then be relocated.