I had Black Lotus, three Swamps and a Forest. I needed to cast Sneak Attack (3R) and preserve red mana to activate Sneak Attack afterward.

I manually tapped three lands and sacrificed Black Lotus for RRR. The legal payment I wanted was three mana from my lands + R from Black Lotus, leaving RR floating.

MTG Arena automatically prioritized the floating RRR from Black Lotus for the generic portion of Sneak Attack's cost, even with Auto Tap disabled and Hold Full Control enabled.

I could not manually choose the mana payment, which prevented me from making a legal game action and directly caused me to lose the match.

Expected behavior: Full Control should allow the player to manually choose which available mana is used to pay generic costs.