I had a similar problem with my female standings. I took her to the vet b/c I was HORRIFIED when I saw her pull of a portion of her tail, part of which was a regen from before I had her. The vet told me that one of the plates in her tail was most likely broken and irritating her and so she bit it off. She didn't eat it though, just dropped it. This may be the problem with your gecko.

If it happens again, you might want to see a vet.
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Lisa

1.0.0 P.m. grandis (Linus)
4.3.0 P. standingi (Toby & Kona and their 5 babies)