When I try a “Heal” in developer.athom.com on a Z-Wave device, the feedback is always “REQUEST_NEIGHBOR_UPDATE_FAILED” after a set period of seconds.
Heal is done on both devices I have a good connection with and devices that seems to be out of reach. Even if I move the latter next to Homey it still will not “heal” and sends the same error message.
However, you can also heal battery powered devices if you wake them up manually first and then click quickly on the Heal button. I do this with all my Fibaro motion, door/window, flood sensors, Aeotec door/window, wallmotes etc.
Make sure you heal them while keeping them in the final position, no point to heal while close to Homey and then move them away afterwards.