Auto pay adds extra mana to your mana pool when using Urza's lands and Forsaken Monument

[SadClaps]

Extra mana is being added to mana pool when auto paying for mana costs using Urza's land and Forsaken Monument:

In the screenshot I've just finished casting Emrakul for 15 mana, all lands I control are visible, off-screen there is a Forsaken Monument. Running the calculations, I should get 3{C} from the Mine, 1{G} from the Planet, 4{C} from the Tower, 2{C} from the Nexus and 6{C} from the Workshop, for a total of 1{G} 15{C}, enough to cast Emrakul and be left with 1{G}. Instead, I'm left with 8{C} extra in my mana pool.

From a bit of testing this does not seem to happen if I were to manually tap the lands for mana.