This can happen if you’re using OrcaSlicer instead of BambuStudio to generate the .3mf file but I guess you’re using Bambu Studio.
I also had this happen to me couple times at random. One fix that worked for me was to start a fresh project, import model, copy over settings so things are identical between the two, hit “slice” and then save. Then MakerWorld seems to be happy with the file, despite both rejected and accepted files being identical…