Check https://www.youtube.com/watch?v=LcmBhm9C7j4
If you pause at 16:20 you see that this category XI biter health going up again.
If you pause on 40:20 at the right frame, you may see a category XI biter with 105% laser resistance.

Is this intended?

I have the same query - I'm looking at the category XX biters in creative, and they have >=100% base or modifier resists on most things. I don't mind having enemies that are tough to kill, but I'd prefer them to be technically capable of being killed.

I just laid a XX biter in a pit of flames, let them whale on it for 20 minutes, and it had dropped down about 10%. Then dropped 20 nukes on it, and the >100% resists meant it repaired back up ...

Jeez, is this a God critter?

I wonder if this is a game bug? Unless the mod adds resistance x critter_level without a cap to 95%?

possible solution: add a setting to say what the max resistance would be

Thanks for reporting. (I was off from Factorio during weekends.)
Should be fixed in 0.17.9 / 0.18.1.
A new asymptotic formula is applied, so damage resistances approaches 100% slower when closer to that, and never exceed that. Low resistances (esp. in lower tiers units) are unaffected; while high resistances (e.g. category XX) can have 99%.

It is an interesting finding, BTW. The formula were made, when there were still only category IV to X. When extending to category XX, I expected the game to treat it like 100%.
Such unintended behaviour has immediately brought me some potentially "funny" ideas in my mind:
A/ Maybe a species of fire-affinity lifeform, where they absorb fire? (Something like a volcano dwelling creature, up to a fire elemental in fantasy lores?)
B/ How about a venomous lifeform that actually like poison a lot? Spitting poison all around when fighting. And the poison puddles damage players but heal themselves at the same time? (Not to say poison capsules and poison clouds would only be beneficial to them...)
These can be quite wild, and not fitting the theme of this mod. So if I ever come to them, they will only appear in other mods (like Schall Alien Mutation or a totally new mod. Never be any new species in this mod.

Thanks! I really, really appreciate this :) And yes, I think there's definitely scope for using the >=100% mechanic in something like Shall Alien Mutation, but as something you opt into on purpose!

thx for the fix, special life forms have promise but i think your right they should be a different mod