I did all of the platinum challenges but after finishing defence I'm stuck in the plat zone. It might be because I don't have enough P's as I only collected 4 in some of the challenges but this shouldn't matter as stated in the rules. I would show a screenshot but the site didn't let me

Can you clarify which mode you selected at the beginning of your playthrough? If you are playing the map normally using the hub world, you will need to collect every P in every level to progress through each zone. If you chose speedrun mode at the start of your playthrough, the map will teleport you from level to level and only requires you to get 4 P's in each level to progress. If you did select speedrun mode, it definitely sounds like a bug.

Yea I chose speedrun mode and when I try to go through the pipe there's an invisible floor that I can't go through

How do you think I could fix this

I saw that you @SimpleShark in the discord. The best thing you could do is record it happening and post a video in the discord showing it for shark to see.