As someone who played BG1&2… I agree. Now, I did enjoy Jaheira and Minsc - they were really well
done. It am more mixed on some of the other returning elements, but the biggest issue is what you pointed out - act3 feels like departure than continuation.
Companions and returning NPCs get sidelined in favour of new/cast.
However, I am unconvinced if it response to fans complaining it is not BG enough, or just consequence of player freedom - or maybe combination of two. Introducing new characters and content means that no matter what player did before, they will have something new to do when they come to Baldur’s Gate.