Monastery Swiftspear Bug
i cast thud on monestary swiftspear when it was already at ⅔ intending for its prowess trigger to pump it to 3 to deal 3 dmg to the target. but the prowess trigger ordered itself after thud resolved causing the loss which since the event is single loss cost the entire event entry.
WotC_VNDR_Jessie#07208
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Rezzahan#77802
commented
Thud requires the sacrifice of a creature as additional cost. Since that sacrifice is part of the cost for casting the spell, it happens before any triggers from actions taken during the casting process can go on the stack, including prowess. In fact, the creature is gone before the spell beomes cast, so prowess on that creature wouldn't even trigger.
In short, Arena worked fine, YOU messed up.
If there was a prowess trigger from the sacrificed creature, then that is a bug.