As I understand BGT forums are dead, hail EE and all that... so sorry for offtop. Probably fix for my problem could be included in future Fixpack or there is some easy solution that i am not aware. Sorry for my english level etc.
So the bug is that if you create custom hero in BGT BG1, damage rolls against him will be completely messed up. Shank (assasin in a house near the stables) will inflict 4 damage when its capped to 1 in original (its tutorial after all). I checked BG 1 original and EE - he should inflict exactly 1 damage every time and 2 for criticals. But with custom protagonist damage will be 4-5 which is exceeds dagger parameters.
The same for stable-guy with staff. He inflicts 8-10 damage every time when he should have 3-8 range considering 1-6 base damage, +2 from proficiency and no STR bonuses. Once again I checked BG 1 and EE - everything is fine there. If you take them under control with CTRL-Q, their damage will be normal, so this extra damage exists only against our hero.
This two NPCs are the easiest to check the bug, but the problem persists for entire game resulting in main hero getting oneshotted even from cobold-archers.
I guess this is happening because during character creation in BGT the game considered to be BG2-like. But since its BG1, something strange happens. If you choose premade BG1 heroes (labeled as BGTHIEF, BGMAGE), instead of creating your own, the problem will be gone. And with Shadowkeeper you can easily edit premade-monstrosity into your own character.
But if you choose BG2 character (labeled as THIEF, MAGE) the problem remains, proofing that its engine problem during Trilogy installation.
Installing TobEx or Fixpack before Trilogy doesnt fix that.
Looks like EE version fixed that, but in my recent playthrough with some mods and tweaks (humans are able to multiclass) this bug happened again.