I just played an Infectious Sporeling (“After this damages a minion, transform it into an Infectious Sporeling”) against a Warrior. He played a Bomb Wrangler (“Whenever this minion takes damage, summon a 1/1 Boombot”). When I ran my Sporeling into the Wrangler, it transformed it before it got to summon a Boombot.
I think this is a bug because “Whenever” effects should trigger before “After” effects, so my opponent should end up with one Boombot, not zero.
Looked into this a bit, because I was curious about this point of phrasing on some other similar cards a while back. From what I can tell, the word “After” on Sporeling is a lie. A more accurate phrasing would be “Whenever this non-fatally damages a minion, transform it into an Infectious Sporeling.” It uses the same phase as most damage triggers, but they’re apparently using the word “After” as a cheap way to suggest that the recipient must survive. Not too surprising. “Whenever” and “After” often get mixed up haphazardly for anything other than playing cards and summoning minions. This one will resolve with the same timing as “whenever” triggers, so if it transforms them first, the other trigger gets suppressed.
I did say “if”, because the order can vary. In cases roughly as rare as your opponent knocking your Sporeling into its neighboring Acolyte of Pain with a Supercollider, it’ll come down to order of play which one triggers first and whether or not you get your card draw. More often than not though, the Sporeling and the thing it damages and triggers will be on opposite sides of the board, which causes the Dominant Player Bug to decide things.
The Dominant Player Bug is a long standing issue that states that when two triggers on opposite sides of the board react to the same event at the same time, Player 1’s trigger always goes first. Player 1 is randomly assigned, so when your Sporeling attacks the enemy Axe Flinger, whether you’ll get an axe flung into your skull is mostly unpredictable. If you’re playing with the AI though, the human is always player 1, so it’ll always be the human’s trigger that goes off first when a Sporeling and another minion collide.
More to the point though, there are damage triggers that actually do go off in a separate phase, after damage is dealt. “After this survives damage” minions like Grim Patron don’t seem to exhibit the same behavior. If I have a Patron on my side against an AI’s Sporeling, then even with the bug ensuring my triggers in the same phase go first, the Sporeling will always transform the Patron before it can spawn another.