ImageService published from Pro with classified raster function, transparent color becomes white

[RichardReinicke]

Hi,

I'm publishing a mosaic dataset with a raster function from ArcGIS Pro 2.8 to Portal 10.9 federated server as ImageService and register the service to the portal.

If I investigate the mosaic dataset with attached raster function in Pro I get the correct classification with the first class as transparent.

But if I load the published image service into portal map viewer, the first class becomes white and I have to set it manually to "no color". 

My overall process is completely automated so this is no option.

How can I get portal to understand the first class color map to be "no color"?

Thank you for your help!