UI Image changes to black after rescaling it if I'm using lights on diffuse material

I have a dark cave area in my game and there are some enemies with health bars on the top of them. The bars are set up on the canva in world space. The whole area is supposed to be dark so every sprite is with Sprite/Diffuse shader and I illuminate small area with light. The same goes for those health bars on UI, I set proper material with proper shader on them. Everyt$$anonymous$$ng looks fine but until I scale a green bar (that is in front of red bar) when enemy takes damage it changes to black. It's not that the color gets set to black but it just stops rendering anyt$$anonymous$$ng on t$$anonymous$$s image, it's really weird. Everyt$$anonymous$$ng works great when I use normal sprite shader for the rest of the game. Any help?