First rule out the oil. Did you do the original service. Was it the correct Kubota UDT oil?
If you have the oil that came out of the tractor before your service Take a good look at it. Put it in a glass or white open container, let it settle and it may give you an indication of what is causing the filter to clog.
Don't know if you should be able to see through the filter. I serviced mine lately and don't recall if I could see through it. Did not have difficulty establishing that it was clean.
Can't tell from the photo. Are there deposits on the filter? Are they black? Do you have compressed air to blow deposits away.
An oil and filter change is the obvious way to go.
Pressure testing (covered in the manual) can be done at various points around the HST. You need a hose, gauges and fittings.
Hope the HST is not damaged, Good luck and let us know how you get on.