ReShade disables depth access in multiplayer games (i.e. when ReShade detects network activity from the game) to prevent cheating. So each time WoW produces traffic, your depth buffer is disabled. Nothing you can do about that since custom ReShade builds that don't contain this safeguard don't get whitelisted by anticheat software.
Posting for other wow players to see, my issue was:
in-game wow options - video - effects - full screen glow effect, located at the bottom.
Although now the depth field is constantly black but at least not moving/flickering, not sure if I have 3D now, hmm.