Here's a draft you can paste into the bug body field:

Summary:
Advancing the Spirit's replacement effect ("You may pay {0} rather than pay the power-up cost of the first power-up ability you activate during each of your turns") did not apply when activating Shang-Chi, Martial Mentor's power-up ability. Instead of being offered a {0} cost, the game required me to pay/tap mana for the ability as if Advancing the Spirit were not in play.
Cards involved:

Advancing the Spirit (MSC #712) — in play on the battlefield
Shang-Chi, Martial Mentor (MSC #732) — power-up ability is {5}{G}{G}: Put three +1/+1 counters on Shang-Chi (doubled to six by his own static ability)

Expected behavior:
Since Advancing the Spirit was in play and this was the first power-up ability I activated that turn, the game should have let me pay {0} instead of {5}{G}{G} for Shang-Chi's power-up ability.
Actual behavior:
The game tapped all my available mana to pay for the power-up activation, as if the {0} alternative cost from Advancing the Spirit was never applied or offered.
Steps to reproduce:

Have Advancing the Spirit in play.
Have Shang-Chi, Martial Mentor in play (or any creature with a power-up ability) with no other power-up ability activated yet this turn.
Activate Shang-Chi's power-up ability.
Observe that the game charges full mana cost instead of offering the {0} alternative cost.

Impact:
This effectively makes Advancing the Spirit non-functional for its core purpose and cost me significant mana (and tempo) in the game where this occurred.
Platform: Windows