Return of Reckoning

So we have Ancestors Fury and Inspiring Attack, both boosts STR. But effects not stack, is this bug?

No, that's not a bug.
Skills don't stack with each other like everything else in the game (skills stack with tactics and potions).
You have access to two stat buffs.
Depending on your spec, one of them will be quite high, around 140.

Tnx for answer! I understand, but first skill costs grudges, so i thought it may be addition, not replacement

Simple way of looking at it is skills and tactics dont stack with themselves or each other. The strongest buff or debuff of its type takes priority. This is true for all skills and tactics in the game.

The general stacking rule is u got only highest from skill+tactic+moral and nowadays liniment/stat pot
Sadly there are some exceptions added by ror devs so best course is to test it out