Bonk's been having trouble and I wasn't able to find anything exactly like what I saw with him online, so I'm just posting my experience. Maybe it'll help someone?

He's about 9 now, eating normally, staying in his hide a bit more than normal, but still out and around. I didn't see his toe for a while, since he's been tucking his hands in. Also he doesn't seem to want to grip with the toe.
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The vet got an x-ray and said it didn't look like an infection, but like a mass. The lumpy toe looks like there's some slight bone loss there.
Possibly cancer, so if it gets worse the only option would be amputation. Could be benign, though.

Anyhow! Weird lumps on frogs happen and it's not always easy to tell what's causing them just by looking.