Hi rross,
We're sorry but currently Bluray discs and in turn Bluray players do not support XAVC S files. That is why the videos are being converted to AVCHD format and plays back at 1080 at 60i. If you want to view the videos on your 4K TV, then I would suggest playing them back on the camera and connecting both devices via HDMI. or you can also export them to an external storage media device.
NOTE: In copying movies recorded in XAVC S, use a recording media which is formatted in NTFS.
If my post answers your question, please mark it as "Accept as Solution". Thanks_Mitch