Pretty sure I've killed the end boss, but when the cutscene isn't loading. I've jumped into the head but it's no solid like I've seen in other examples on YouTube. Anyone else encountered this glitch?

Are you talking about killing the Nihilanth and the following Gman speech that ends the game?

just stay the course. that realy bugged me too when he keeps teleporting you to these wierd places and shit jus wait when you do what you ave to and you go back you have to get up to the highest ground destroy te little triangular lights all around the rom and eventally his(?) head will open up for longer and longer util he dies. (also after that when u get the chance walk out of the train car trust me u will want to in the end.