Hey ! Just read your message and edit. I think that's because you went to the alternate "hardcore" section of level 1, which I did not register on this video. I can do so if you want. To confirm : after the first dialogues, did you go underneath the starting platform/island instead of going right ?
If so, you didn't go to a non-planned part nor got lost. You went to the hidden path for hardcore players. No-one mentioned it so I thought people just didn't find it and it wasn't important.
I don't remember precisely the steps I took before that point, to be honest once I realised the jumping mechanic I spent quite a bit of time just flying around exploring so it's absolutely possible! I remember moving onto the sand timer section, and the slightly different coloured underground section? Not entirely sure which order I did those in... but I think I unlocked the wall jump in or just past the bit I felt was underground.
Hmmm ok so no that's not it. I have no idea where you got lost ><
I don't know eithher how you could get to this level without unlocking the mushrooms. I'll have to try to do what you did to find out, my guess is you jumped on the upper platforms somehow to go directly to level 2 (the pink level) without really playing much into level 1. If you couldn't use the mushrooms jump after then you couldn't finish level 2, that makes sense. I thought I'd made things so you couldn't go to that upper area in level 1 without unlocking the mushrooms but I probably didn't think about everything people could do if they didn't follow the path I'd set.
