Could just be a bad habit.
My first mare, I picked her up from her previous owners, and on the way home we stopped at the tack shop, tied her up outside, and she took some big chunks out of their wooden fence.
Brought her home, gave her all the hay she could eat, grained twice daily, rode 5-7 hours most days (nice being a high schooler...), and she kept eating the trees.
Got her a friend, and she still continued eating the trees.
Put chicken wire up around all the trees in her field, which helped, but she'd push the chicken wire out of the way to continue to get at them.
Painting the no-chew stuff on didn't do anything at all.
Cribbing collars and such don't help for chewing.
When I later had to board her, I HAD to make sure anywhere I took her did not use wooden fences.
She never stopped chewing wood until the day she passed on, no matter what we tried. It was apparently just part of her (kind of like I will probably chew my nails until the day I pass on