Yes, a RAW-to-DNG conversion can introduce problems if the converter or the software reading the DNG has a bug or misinterprets the file. What you’re seeing is unlikely to be normal "degradation" from DNG itself, especially not such extreme color shifts.

In general, lossless DNG conversion should not alter the underlying image data in a visible way. Severe red/green corruption points more toward a conversion or rendering issue than ordinary quality loss.

Since the original X3F files render correctly in Sigma Photo Pro, the safest conclusion is that either Adobe DNG Converter or the software decoding those DNGs is mishandling some Sigma files.

Practical takeaway: keep your original RAW files, restore from backup if needed, and avoid converting to DNG unless you have a clear reason to do so. If your original files work properly in the manufacturer’s software, they remain the best reference for correct rendering.