I would say: the author of the score on musescore.com intended the notes for playback by a human, and did not care to hack around in Musescore to get its playback exactly "right". ("Right" as in, to sound like they would play it themselves, or like in the original recording they did the transcription from.) The score in Musescore is a bit different from the one in the "OST" Youtube video, and the program's playback is probably not even what was intended. I can't hear much of an arpeggio on Youtube.
This is what it sounds like on Youtube (relevant part at around 2:20):
This is the playback in Musescore 2.3.2
Would any human player interpret the notes anywhere near like that? If the Youtube link to the original sound track is what was transcribed, then Musescored playback is plain wrong. Even the transcription is inaccurate, because I don't think there are any arpeggios. Maybe a different original soundtrack version was used for the transcription.
Replacing the grace notes with acciaccaturas changes the Musescore robot's programming, try it and you'll see. You could also add arpeggio lines, which are wavy lines in the "Arpeggios & Glissandos" palette.
See Different types of grace notes
Apparently, getting Musescore's playback to sound exactly "right" (i.e. in some particular way) needs some hacking around with hidden markings and plugins, to play back things like ritardando
For comparison, here's how Sibelius 6 interprets the same score, opened from MusicXML format (and after resetting layout and spacing). This is very close to how I would play it myself, except that the "rit." marking should be some kind of slowing-down (ritardando or ritenuto), but it doesn't seem to do anything.