Answer
If you're on Apps10 UPD10 or an earlier update level, this specific OpenAPI v3 documentation-generation error for custom projections has a confirmed permanent fix delivered in UPD11 — upgrading to UPD11 resolves it outright rather than requiring an ongoing workaround. If upgrading isn't immediately possible, this was a known, previously-discussed issue in the community with some interim workarounds documented in earlier threads on the same topic, worth searching out specifically if you're stuck on a pre-UPD11 level.