For GeoTiff the raw values are exported. Without the scaling and offset of the band.
You can either look up these values in the Information Window for the band and apply the values on the exported GeoTiff. Alternatively, you can, before doing the subset, create a new band by the band math.
As expression simply use the name of the TSM band. I’m not sure if really necessary but it is better if you ensure that the Virtual option is disabled.