I think the source of healing isn't taken into account. I *think* I remember mend wounds healing me less when I was affected by stoneskin and shield, as a shaman.

What counts is where the damage reduction came from. For instance, if you can commune sanctuary and are affected by sanctuary, then it doesn't mess with healing. If you can cast corporeal hardening and are affected by corporeal hardening, then it doesn't affect healing. Etc.

At least, that's always been the rule of thumb I used.