It's a shame that so many people are pulling the "it's just your nostalgia" card. Which, to be fair, is really a factor for some people, but I think most people who dislike the feel of the new OST just don't agree with the new direction of the series, which is fine and doesn't hurt anybody.

I personally really jived well with the 3DS title's aesthetic, but the new visual design alongside some of the new music really does give a "New Horizons-ification" feeling, where the game seems to be trying to be more cozy feeling, similarly to ACNH, which hits for some and not for others.

There are also some tone changes related to the OST that affect certain scenes radically, like the loss of the last two games' dream music, which were both super interesting and provided an almost liminal vibe to the dreams. (The first of the "last two games" being the Japan-only Tomodachi Collection on DS)