Mana Auto pay mechanism issue
When I manually tap lands before casting a spell for exactly the amount and the color of mana that I need, I expect the spell to be cast smoothly. Yet, in the scenario I'm gonna describe, the game doesn't accept the black mana generated by me, and enforces Auto Pay, to re-generate it again using my remaining untapped land. That's quite annoying and I keep losing because of that :(
Context: Android app
Scenario:
I want to cast Tezzerrt's Touch. Before doing that, I manually tap my lands explicitly for: 1 🔵 blue, 1 🖤 black and 1 ⚪ colorless.
Expected results:
Tezzerrt's Touch is cast smoothly on the target artifact untapped land I control and that lands remains untapped!
Actual result:
The game doesn't let me cast Tezzerrt's Touch, but instead offers me to Auto Pay for it. Why? There's no way I can avoid this, so I have to click it, otherwise the timer will run out. After I click Auto Pay, it still DOESN'T make use of the mana in my mana pool (at least, not the black 🖤 one, for some reason), but would re-generate it ADDITIONALLY using the remaining untapped land I control. Only then the spell is cast. As you can see, on the second screen I'm left with 1 extra 🖤 mana left, that I didn't need, but was forced by the game to generate :(
Is it a bug or I'm stupid enough to not see something obvious?
(P.s. am not a noob mtg player, so any questions - let me know!)
Regards,
Igor