Didn't noticed before but it's true. Probably haven't tried it for months or years, the only mob you need to walk backward from doesn't need healing at all
It's an old feature, you can heal when moving when you turn on "walk" mode.
If you walk downhill the game thinks you move faster and you get less heals. Also, walking doesn't always get you full heals so it's still best to stop briefly.
At first i understood this as: "i can get full hp heals when walking backwards."
and thought - great now we have mod fap heals for everyone .
dumb-but.
After understanding properly what you ment:
its not a bug i think. Its been here for years and does not give much advantage to be honest. There is speed limit when you can still have normal heal instead of "reduced due to movement" waling any direction not only backwards.
Oh wait, I read the op wrong. Healing while walking backward is indeed an old feature, but what I wrongly thought and what I just tested to be true and that healing is now diminished for me while walking backward (but not forward).