I have no screenshots, but I just had a game where an opponent played Stoic Sphinx. Since he was playing a blue deck, I baited out a counterspell on my turn to make it lose hexproof and kill it with fell (sorcery) on my turn, since the card states that it only has hexproof until the owner has played a spell this turn. The cards owner did exactly that, however the card did not lose it's hexproof ability for the rest of the turn and hence could not be targeted. Sounds like it does not work as intended.

I feel like generally it did not lose hexproof, no matter how many and which spells are played a flash creature spell on my turn from the opponent also did not make it lose it.