Storm the Vault won't flip when it should sometimes

stikxs

I was playing the Across Ixalan event and thought I noticed something on my first match but wasn't sure. Second match:

I cast a Spell Swindle followed by StV, which is guaranteed 5 supports to trigger the flip yet nothing happened. I believe I also had Millstone and Rishidan Port out too (thought the Port wouldn't count to flip StV). I went another turn and it did not flip (I believe I had the same supports, though could have lost a treasure, but the game also creates treasures at the start of turn). It wasn't until 2 turns later that StV flipped in that game.