Feeding a lightbug, usually after swapping backpacks, causes aberrant movement in the character and will not allow you to feed the lightbug unless it's in your personal inventory.
I've not tested with other creatures you can feed in inventory like moleworms, rabbits, birds etc.
Have Bulbous Lightbug in backpack.
Have compatible food item in backpack.
Open inventory, pick up food item, attempt to feed to lightbug.
Character will behave erratically, moving in circles, trying to run in a direction etc. (this behaviour is similar to another bug, when interacting with a chest that drags you from where you were standing)
Cancel out of the animation if you can.
Move food and lightbug to personal inventory.
Feed the lightbug and it will be ok.
If this doesn't replicate, swap backpacks and reattempt.
If that doesn't replicate, swap backpacks at sea, and reattempt.