Yeah, this is just my sperging on Dylan’s vocal strain. The key he is singing in is the biggest issue, as when it gets to the higher parts near the end, he is struggling. If you look at the video, his eyes seem to be popping out of his head like he isn’t anticipating those higher notes. Also, with higher notes, the mouth needs to be shaped in a certain position to achieve the sound safely. Dylan doesn’t open wide enough or leave enough space in his mouth to raise the soft palate, so the sound ends up a bit flat.
The ill fitting key also creates vocal control issues, so in the video, you see the high notes sounding louder regardless of the lyrics. The song itself is very mellow, so it’s unpleasant for the listener to have those high notes be extra loud in comparison to other elements of the song. The lyrical interpretation of this song would be much better if he just sang at a lower key. Billie Eilish doesn’t even have a super high voice herself, so I don’t get that decision of his to sound even higher than her.
Tl;dr the improper key, leading to more falsetto use, is giving Dylan vocal strain because he isn’t trained well enough to sing in it. Although, singing falsetto isn’t a requirement to be a good singer.