I've run into this basically throughout the game from lvl 1-45+ Pve..

you'll have a group of mobs that are known to be vulnerable to X-damage type but some of them return as neutral.

Some Animal types-say Wolves and badgers behing NBeo return as neutral to slash and or Pierce damage. etc

it usually happens in 'packs' of mobs. Generally solo mobs wandering follow normal tables, for instance:
some bettles outside of MM will return as neutral to blunt damage, spiders in Muirie. In CM Silvermine Guards, Gemclickers and Hoarders-all known to be blunt vulnerable return as neutral. In the Pit some Weewares, Werewolves are neutral to slash/pierce, when a pull before you get (+) to damage. As are some of the Vein golems, and protectors and runnners.

My question is do you need screenshots or video? I'm willing to provide either

You must give exact names of these mobs. This is worldbuild and must be set per mob.

Muire Tomb

Crypt Spiders - some are blunt vulnerable some are not
carrion scorpionida - some return as blunt vulnerable most aren't
Scorpionida Regina - blunt nuetral since its a 'scorpion' it should return as vulnerable -although since its a 'named' mob this is perhaps by design?

Water Beetles/collectors south of Mag Mell and North of Ardee- some aren't Blunt vulnerable

All of the Land Sherrie mobs, should be vulnerable to blunt yet some return nuetral

CM mobs

Gemclickers - since they are a 'shelled mob' should be blunt vulnerable some are not

CM Pit -
Silvermine Sentries/Guards - most are except the high level one (OJ to lvl 46) at the top of the pit he returns as nuetral to Blunt weapons

Vein Golems, Load Protectors, Load Runners- All should be blunt vulnerable but only a few return as such'

silver-maddened werewolfs - should be vulnerable to slash/pierce a few return as nuetral in both camps at the pit

Lobigan - nameed Silver-maddened werewolf- returns as nuetral to slash/pierce damage-again named mob could be by design?

I made a pass through CM and corrected about a dozen mob types. Muire should also be better. Please let me know if you notice any more in there without proper armor types, and feel free to locate other specifics as well. I do specifics like need names and locations rather than "all spiders", for example. Thanks for the report!

Out of curiosity, I thought that mob spawn is one point and mobs are randomly spawned around it, or do you in fact have to set each mob separately?

each mob is seperat in dol afaik and simply roams around in a small field or doesn't

Villanous YOuth mobs in camp with 3 tents and some barrels,just behind Ardee (S/SE direction from Ardee),have strange melee resists/vulnerabilities. It changes from hit to hit,or few hits.

This happens on live too where the damage or resist is rounded.

It made testing level 1-10 exceptionally hard for resists ;(

ok,thx for answer.