Yes, I understand it's not just the invisible zero neurotoxin counters.
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AI doesn't take any Neuro damage or effect from the original Dune
Actually, I'm confuse when you say this. In point 5 you mentioned that the AI indeed took some damage while there was no counters visible. Or did I get that wrong?
I see it as a visual bug because the damage happened normally but there was no counters visible. To further clarify what I'm trying to say, let me try to explain using your example:
1. I apply Neurotoxin with a Blessed Dune. OK
2. AI plays Purify to get a Purify counter of +2. OK
3. AI doesn't get any more Neurotoxin counters by playing new cards and doesn't take any damage OK, purify played. No neurotoxin.
4. I apply Neurotoxin with the Blessed Dune for two turns, bringing both the Purify counter and Neurotoxin counter to 0. Normally this is when you'd see the yellow Neurotoxin counter at 0, but instead there is no counter at all. AI got neurotoxin'd again. First visual bug: counters disappear.
4.5 Subsequent turn(s), AI still has neurotoxin because no purify been played. Damage is taken and it will increase each time a new card played. Second visual bug: Counters still invisible.
5. Here's the weirdest part: by playing a new Blessed Dune, the AI gets the Neurotoxin counters back, but they start out at the amount they should have considering all the past hits. The counters suddenly appear again and it is the correct number.
6. Repeat from point 2 if necessary.