No, I was never able to resolve this issue, but may have identified the source of the problem...
After resetting the TV to run full Android, I installed Kodi. Under System Info > Video, the Mali-G52 GPU reports the same specs to Kodi running natively on the TV as I see on a Roku using one of the 4K HDMI inputs: 1920x1080 @ 60.00Hz, HDR10, HLG, Dolby Vision.
Despite this incorrect resolution data, 4K videos still play correctly in all the native Android apps that I have tried.
The Roku seems to (correctly) see this 1080p spec, but then won't let me force an override to 4K with Dolby Vision. So, while the problems is on the Hisense side, it has forced me to abandon the Roku platform. Hopefully someone in vendor relations at Roku can help get this sorted.