I don't know if it's intended, but Light as a Feather does not remove Lumbering caused by Weighty. Trees are sad.
Yeah, I think the fix is that when a champ gains Flight it loses Weigthy and Lethergic. If the champ has Lumbering, it gains Weighty instead.
The solution that would cover every possible way a champ can become Lumbering is to make the champs affected by Light as a Feather ignore Lumbering instead. Works on Weighty. Works on Lethargic. Works if the champ had Lumbering when cast. Works if the champ gained Lumbering after cast. Might as well future-proof the ability while its being re-programmed.