I know a little on African Bullfrogs, but I am no expert. What I would do is try introducing new foods. I had a pacman that ate crickets, and then totally stopped eating crickets. So I gave him nightcrawlers, and he loved those.

If you feed him in the substrate, he could accidentally ingest some substrate. Tree feeding him in a small plastic tank.

Your frog does not appear underweight, but what I would do is decrease the humidity. 80 percent is way to much, especially because these guys come from a naturally dry region of Africa that gets little to no rain.

If none of these suggestions work, try a warm bath. If that does not work, I would advise getting your frog to a vet to see what the problem is.

I hope your frog gets better.