Advantage and disadvantage ... stacks, so if your enemy is on highground with Blur, applying truestrike alone will not remove your attacking disadvantage.
Is this true? In PnP it doesn't matter how many sources of advantage or disadvantage you have; if you have at least 1 of each, the attack is made at normal.
I'm actually not opposed to BG3 working this way, as it would help make such spells like Blur, Faerie Fire, etc more useful.
The "one cancels many" aspect of the 5e advantage system is definitely it's biggest weakness. So long as stacking does not lead to "double advantage" or "triple disadvantage" etc, then I would call it an improvement.
I can't say that I have been able to observe consistent behaviour in the advantage system of BG3 EA, so it would be nice to know how it is *supposed* to work ( according to Larian rather than 5e ).