What version of macOS are you using? Which GPU do you have?
I tried Catalina and Big Sur with a 3 port DisplayPort 1.4 MST Hub that should be similar to the CSV-7300. The MST Hub is connected to a W5700 that is connected to my Mac mini 2018. I connected 3 4K60 displays and they all worked as expected (they are all mirrored because macOS doesn’t support MST for multiple separate displays).
The AGDCDiagnose output says DSC is not enabled. I suppose this is expected since the displays are mirrored by the MST Hub and not by macOS (only one of the displays appears in macOS since macOS doesn’t support MST for multiple separate displays). Only one 4K60Hz stream is sent and the MST Hub transports that to all three displays.
I suppose one thing to try is to check the firmware of the CSV-7300. Send an e-mail to [email protected] to see if they have a firmware updater.
You’ll need Windows to upgrade the firmware of the CSV-7300.
There’s a firmware utility at https://kb.cablematters.com/index.php?View=entry&EntryID=147 which will tell you what your current firmware version is.
Maybe the issue is with your GPU – it is probably similar to the W5700 but not as powerful.