Pink Ward not revealing Stealth Wards

In the attached photo I placed a Pink ward, there were two enemy stealth wards in the exact same bush but were not revealed by my pink. I know this because I saw them put them there (hence why I used my pink) and I had a friend spectating who confirmed that there were indeed two wards in the bush next to my pink.

I also believe that their stealth wards, were somehow not seeing my pinks, because they didn't destroy it for quite a long time. Had they seen it (which they should have if the wards were working properly) it I'm relatively certainly they would have cleared it; as they did not for quite a while and had ample opportunity.