I had a Delighted Halfling and a facedown Rattleclaw Mystic in play. I turned Rattleclaw Mystic face-up (the auto-tap used both of my red sources for some reason which is not the issue I'm reporting but still stupid when I had multiple blue and green sources). I then proceeded to cast Loot, the Pathfinder. I wanted to use Loot's red activation (deal 3 damage), so I tapped my Delighted Halfling to pay for the red mana in Loot's casting cost (so the red from Rattleclaw Mystic would still be available after Loot entered). However, no matter how I tried to do it, Arena was automatically using my generic red mana to pay for Loot's casting cost, leaving the conditional red mana from Delighted Halfling in my mana pool -- this would not be able to pay for Loot's red activated ability.

I don't understand why this would happen, and it effectively cost me the game -- both because I wasn't able to make the play that I wanted to make (and should have been able to make) and also because I timed out due to repeatedly trying to get the game to work the way it should. Log file is attached. Please fix your game, it's ridiculous that MTGO, an ancient piece of software, can correctly model the rules of Magic, and Magic Arena can't.