I've recently updated all of the SolidWorks, PCB Connector and Altium seats to 2020 and no all the solder mask decals wont show semi-transparent. Before the solder mask decal would replicate the solder mask as a somewhat see through layer over the copper decal. Now when I open the board the mask is solid and looks like a gray sheet over the copper decal. If I change the transparency option the decal only gets lighter or darker not transparent. It happens on every PC and laptop I have upgraded. It is also happening on fresh installs and it doesn't help to regenerate the files out from a new or old push out of Altium. The only image I have of what it use to look like is the thumbnail embedded in the file so its a bit grainy but you can see how the pads of the board are bright until they are under the mask.
I've since tried to apply one random decal over another and see if I can make the one on top semi-transparent and I get the same result in that it just makes the decal dimer not transparent.