Reproduction repo: GitHub - benwoodward/dgraph-js-http-upsert-bug: Reproduction repo
Looks like the same issue was fixed in the Golang client: startTs mismatch when running multiple upserts within one transaction · Issue #136 · dgraph-io/dgraph4j · GitHub
possible related: Can't run multiple mutations in same transaction · Issue #113 · dgraph-io/dgraph-js · GitHub