Game Still has Errors with Inherited Abilities
In the MTGA Tinkerer's cube, I had previously created a copy of my Opponent's Defiler of Vigor using Mythos of Illuna, paying the RG to gain the Fight ability. Later in the game, I copied my Defiler of Vigor using Quasiduplicate (which also had the Fight ETB ability), and targeted my opponent's Tireless Provisioner. In response my opponent flashed in Svella (creatures could be flashed as Opponent had activated Arlinn's +1) to trigger his Defiler and cause our creatures to trade.
However, instead of my new Deflier trading with 6/5 Tireless Provisioner, instead the newly summoned Svella fought the Tireless Provisioner. My guess is that the game lost track of which creature was supposed to be fighting and simply used the most recent creature to enter the battlefield.
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Choram#68443 commented
I did some additional testing against Sparky with the following observations.
i) Having a friendly creature ETB with the Mythos fight trigger on the stack causes the fight not to happen.
ii) When using the Mythos on a creature with its own ETB fight trigger (ie. Wicked Wolf), if the Mythos fight trigger is the second one to resolve, no fight will occur. Two fights will occur if the Mythos trigger is stacked to resolve first, with the creature's inherent fight trigger resolving second. By extension, cards which double ETB triggers will only result in one successful Mythos fight (the second one to resolve is lost).