Hi! I think it is not a bug. The character needs, at least, have 2 'levels' of light to unlock the 'light mode'. When you pick up a light sphere, you gain 1 level.
Could be that your problem?
I'm sorry if my game wasn't clear enough. I suppose I'll just write it at the controls tab.
Thanks for your feedback!
* Edit: I just add a new section for the website called 'How to play'.
I hope it helps you and the newcomers! *