[Gameplay] Ward not giving vision

Just played a normal game on blue side. I went to check baron and I put a max range yellow trinket ward in the bush next to blue buff. The vision from this ward appeared as normal and covered whatever radius a normal ward does. However when I walked past the bush to proceed to baron a random Darius and Taric walked out of the bush and killed me. After this they were invisible again so it seems the ward looks like its doing its job but actually there is no vision there at all.

I tried recreating it a minute ago but I couldn't. Not really sure what made this happen, there was a Quinn in game so maybe some problem with the blind or something.