In follow-up blood testing, why would tTG be negative and DGP be positive?

If the slightly positive test is the DGP-IgA, you can disregard this value, as it can be misleading. If the DGP-IgG is positive it probably means you’re ingesting small amounts of gluten, possibly from cross-contamination, which are raising antibodies but not enough to trigger a tTG or a mucosal response. Basically, in either case you shouldn’t worry too much.