Ooh, that's odd indeed! I figured it would be senseless to allow negative regen but not positive. That is silly
I suspect the property was originally created for it's named purpose, but either they decided against it, or couldn't find an achievable way around the side effects.
That was my original theory when this property was discovered - walls have no wreckage and small enough metal it would be hard to get reclaim off of them. I'd love to be proved wrong on this, but I flipped the sign several times and could get it to go down but not up.
It's turned off for now because it was causing errors in TITANS and nothing in the base game was using it. We'll be working on fixing it in the future for modders that want to use it.
When I'm done with triage I'll switch commander regen back to the spawn-unit area-negative-damage hack, but that is all I know of.
If it was possible to have regeneration drain metal that would also fix the balance issue of reclaiming the unit for free metal. While I'm asking for things, stuff like this could also use an in-combat toggle. Dawn of War has a system where units regen outside of combat, but not while fighting so it doesn't factor into combat balance. They also have a fun Daemon unit where they flipped it around, he loses health when not fighting because evil
Looks like the issue was positive passive_health_regen applied to dead units that regenerated as zombies. Resurrection is not currently supported in the sim. It may be possible to reapply limited to non dead units. ie no resurrection.
I just tested this out and it doesn't seem to work? I'd love it to be back in for mods (3 years later I'm back on modding GW again, maybe this time I'll release something )