If you take damage from something that armor can block, and then from something that armor cannot block in the next half second, the second source of damage deals less than it should.

To reproduce this bug, wear a full set of fire protection 4 netherite armor. Place a bucket of lava under you BEFORE placing a lingering potion of instant damage 1 under you. Then remove the lava, heal, and wait for the fire to run out. Place a lingering potion of instant damage 1 under you. You will take more damage without the lava.

This also happens with instant damage arrows. A fully charged bow with instant damage 1 arrows will do less damage to someone with full netherite armor than a barely charged bow with instant damage 1 arrows will do.

Normally, if you take 2 damage, and then 3 damage in the next half second, 2 damage will be subtracted from the second hit, making the second hit deal 1 damage for a total of 3 damage taken. If you take 10 damage and then 5, it will subtract 10 from the second source which cancels it out, so you only take 10 damage total. It is likely that this does not account for armor, so if you take 10 damage, but your armor causes it to only deal 3, and then you take another 5 damage, you will only take 3 total damage because the game thinks that you took 10 damage the first time, so it subtracts 10 from the 5 damage so you don't take any damage from the second hit.