I noticed this recently in todays Deadpool daily. I downed a Sentry Fighter with a critical match, and Captain Marvel (3 star) came in and took the downed Sentry Fighter’s place and promptly absorbed twice and gained 4 red AP twice. She had full hit points and then promptly used here Photonic Blast ability.
This has been happening to me since the Bonus Heroes update. Whenever a new enemy appears, and they have a passive that is triggered by damage, it will go off.
Saw it first with Hulk, a few days ago with Storm, and today with Captain Marvel. All 3 times it happened to me was after I used an AoE attack, and all 3 times, the new enemy took 0 damage.
I think you are right, I saw it also with a 2 star storm substitute.
Had the same issue last time she was in DDQ and same happens today: as soon as you down one of the Sentries, Cap Marvel will come in and absorb damage for the downed enemy. And she aborbs multiple times, depending on how much damage you’ve done to this Sentry 
It’s likely down to order-of-operations weirdness again. The game probably does things in this order:
1: Ultron Sentry in slot A is downed; is another enemy waiting to enter for this wave?
2: Substitute Captain Marvel into slot A
3: Does occupant of slot A have any passive powers triggered by damage taken? [Yes, Energy Absorption]
4: Was damage taken by occupant of slot A greater than threshold for Energy Absorption?
5: Energy Absorption triggered
Without another character waiting to enter, on downing an enemy the OOO is something like:
1: Ultron Sentry in slot A is downed; is another enemy waiting to enter for this wave?
2: Does occupant of slot A have any passive powers triggered by damage taken? [No, downed characters have no passive powers]
3: Do nothing.
The correct behaviour would be to make the substitution after step 3 in the first list, rather than after step 1.
This has been an issue for a very long time. I’ve had it happen in DDQ.
It’s been so long, I’ve given up on it being fixed.
Happened again this morning when Than-man’s court death went off…
Just adding my experience to the list. This should be fixed.
One of the oldest bugs currently still in the game afaik. It’s been around for so long I’ve also given up on it ever being fixed.
They’re too busy working on important things like changing the look of the arrows in the Recruit Heroes menu to bother with trivial matters like this.
I’ve seen this too, but I think the problem goes back further because for several months I’ve seen in wave nodes that the next character is shown prematurely in the spot of the character that’s currently being downed.