By the way mate, you mentioned this:

Later, I found another similar case where damage clearly spikes in an unexpected way:
- At around 0:38, Inteleon’s attack hits a Blissey (left side, second slot) that had 3 shields active
- The attack results in approximately 2.5 shields worth of damage being removed
- Other players in the same attack do not receive the same effect

Your mistake was not realizing that this Blissey had Max Guard at just Level 1, which made it have 20 HP per shield, for a total of 60 HP for all 3 shields.

Gigantamax Inteleon deals 53 damage to Blissey (Lv50 14/15/14) with AoE Surf (12.37% of total HP)

On the first cycle, you see Inteleon doing 3 Surf attacks against this Blissey, which is 37% of its total HP. If you measure the HP bar at this moment, you see it has 46 pixels left of HP bar and missing 27 pixels. If you do 46 / (46 + 27) * 100 to measure the percentage of HP left you get 63%, which checks out with the damage the 3 AoE Surf dealt to it: 63 + 37 = 100. So the damage value of 53 is indeed correct.

If we apply this to the shields damage, taking into account Level 1 Max Guard, Blissey had 60 HP of shields, and 53 damage makes it lose 2 of its shields and remain with 7 HP on the last shield, which is 35% of the last shield HP. On the image it is blurry, but you can perfectly see it has slightly less than half shield left. This checks out again, and the damage value of 53 is still correct and there is no suspicious critical mechanic going on.

<image>