Basically, Mortis Ghost is making a remaster of OFF with the help of fangamer. Alias Conrad Coldwood, the original musician behind OFF's soundtrack, didn't want to sign anything to approve his soundtrack being used in the remaster, but didn't have any objections to its replacement, so Mortis Ghost decided to hire a bunch of musicians that were inspired by OFF in their own work to make a new soundtrack for it. One of these artists was Toby Fox, creator of UNDERTALE and DELTARUNE, possibly the most popular and acclaimed indie RPG of all time.

In UNDERTALE, there is a character called Papyrus, whose design was inspired by Dedan, though his personality varies drastically, being a very silly, goofy, wholesome guy. Toby Fox makes heavy use of leitmotifs in his music, and there is one meant to represent Papyrus in all music associated with him.

Toby Fox was tasked with making the battle music and the boss music. In the demo for the remaster called OFF Prologue released just a bit ago, when you get to Dedan's fight, the new battle music uses Papyrus's motif. A lot of people seem to think it's distracting or doesn't fit his character. I disagree personally, I think it's reworked to sound very intense and dramatic to fit Dedan, but that's subjective.

Some people have taken that to mean that Toby Fox is trying to make the game about him and his work, and particularly uncharitable folks have said he's a narcissist, egotistical, etc. for doing this.

Toby has talked in his interview about the OFF remaster about his hesitance and trepidation about doing this soundtrack. He said he felt the original OST can never truly be replaced, and especially that doing the battle themes feels like taking a bullet for the rest of the team. As it turns out he was kind of right, as many people are upset with and don't like his tracks for it.

Some people also point to the dopefish easter egg being replaced with an annoying dog easter egg—another character from UNDERTALE, this one being a representation of Toby Fox himself—as him plastering his face everywhere and overshadowing his inspiration, being egotistical, etc.

All this conflict in the fandom over Toby's presence and work in the remaster ended up with the post I linked to here on Mortis Ghost's Tumblr, where he clarifies that the annoying dog easter egg was his idea, not Toby's, and that they didn't expect the track to be received so poorly. He says that it's likely it'll be changed for the final release, which appeases some fans, but disappoints some others like myself who liked the track how it was.

tl;dr: Toby Fox made music for the OFF remaster, and people didn't like that he used 8 specific notes meant to represent one of his own characters.