any chance it got placed and then removed by the resurgent mastery?

The Resurgent Mastery does seem like the most likely candidate, :)

The block passive never was on the enemy during this attack. I would expect, that Ronda lands it before she attacks, and if she deals more than 25% of his hp as damage it gets removed. How does the game "know" that the attack will do more than 25% hp as damage when the block passive is set before the attack?

It's easy to miss things, especially on a game that can "rush through" debuffs, buffs and attacks. Think about how fast spider 25 moves if you've frozen the spiderlings. :)

Have to catch the random and very fleeting notice that the debuff was removed.

To further answer your question, I'm sure you know the game does the math of each action before it displays it to you.

This is demonstrable from the inability to even pause during the visual display of locked out turns, and even more obvious when you quit out of a match or a battle and the already qeued display of the enemy turns are shown to you before the fight actually shows a defeat screen. The math has already been done, since you don't have a turn in the middle, and the game is just showing you the visual results. This is why things can really get moving fast when champs are frozen and there isn't much to show you visually for a turn, the game just rolls through it.

The notice that a debuff was removed is indeed easy to miss. But the red debuff icon on top of the head of the enemy, placed before the attack is launched, is not. I would have seen that. And especially wouldn't I miss that while doing the fight manually and watching out to see that debuff icon because of the observation I made before.

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Feel free to submit a bug report if you don't agree that it is the enemies' masteries. Her block passive skills debuff is working fine for me.

And here's an obligatory pic of the debuff doing the job against a champ who, due to Block Damage, cannot Resurgence the debuff away....

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And another just for to show it works normally too...

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I don't say that Ronda never lands her block passive. But is it really impossible that they coded like usually and it's possible to resist, even if the skill description says otherwise?

It's exactly like that in Bommal. The passive of this boss says, it's impossible to block or resist the bomb debuffs he gives in return for debuffs placed on him. But still Ghostborn can put decrease def on him without getting a bomb, resisting an irresistable debuff. He just did so in the move before this pic was taken. Everybody has a 2 turn atk buff (not from Duchess, no 2 turn block debuffs on my champs) and Bommal has a 2 turn decrease def. Where is Ghostborn's bomb?

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I think you may want to reread what Bommal's passive does, it's not irresistable, just can't be blocked or removed.

Considering my Ronda has 111 accuracy, which is lower than I thought my account even could go these days.... seems the irresistable debuffs are working exactly as described for me.

Also you are incorrect on how Bommal's Pressure Wave Passive works:

Pressure Wave [Passive]
Each time an enemy places any debuff (except [Bomb] debuffs) on Bommal, places a [Bomb] debuff on that enemy that detonates after 2 turns.
These [Bomb] debuffs cannot be blocked or removed. 

As you can see, the bombs CAN be resisted. They just can't be blocked or removed. 

My main strategy for ALL my Bommal teams are to go in with high resist and never get any bombs. :)

This one I think is because of UDK. Note that Mithrala has the other half of the debuff. I wonder if UDK absorbs the first half of the A3 because, despite the fact that it is "before attacking", it's still part of that first hit? And, perhaps the reason UDK doesn't have the debuff is because that first hit was a weak hit?

Debuffs that are placed before a hit cannot suffer from a weak hit. 

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Thank you for that information.

Please respond to Plarium that their translation team made a mistake and wrote in the german version, that these bombs can neither be resisted nor blocked, while the fact that they can't be removed is not mentioned here. I knew that they can't be removed from the "yellow line" the icon of these bombs have, but indeed it's not written in the description. It seems as if they confused "can't be resisted" with "can't be removed" in the translation.

Not my fault. 😇

Thanks for the info! I'll inform the localization team. Always worthwhile to temporarily switch languages or double check the English descriptions on sites like HH, when you run into such problems. And of course, you can of course always write me when you see any errors.

Either way, it doesn't change that Bommal is working as properly written, at least.

Additionally, I am not seeing even a single issue in my play with Ronda. I would be willing to bet, again, on the simplest answer here being the correct one. You are seeing a very common mastery that triggers basically on 50% of hits actually doing the job it is supposed to do.

If you can record some videos to the contrary, rather than a screenshot after the mastery would have triggered, I'd love to see it.