I suggest you either:

a. Take the machine back to the dealer and perform your pulls in his shop OR

b. Separate the tubing (you need to know which is which) as I described and do the test as I described.

Only then can the problem be further discussed/solved.

I'm not an expert, but I think the problem is due to a faulty pump (again). The machine now has difficulty reaching 12 bars of pressure during back-flushing. What typically happens is that the pressure will climb to about 8 or 9 bars, but then struggle to reach the OPV setting of 11.5/12 bars. From the time I push the button, it takes approximately 30 seconds for the needle to reach the 12 bar marker.

I returned the machine to the vendor for repair, and he's going to install a new pump when the next order arrives. There was a bit of a language barrier with the technician, but I think he said they received a bad batch of pumps as a few issues have been reported. I might choose to upgrade from the stock pump as it appears Bezzera is having some QC issues. I will likely have to wait 2-3 weeks for the repair to be done as it takes some time for the parts to be delivered. I'll provide additional updates as they become available.

This has been an incredibly frustrating experience, and I sincerely hope this is the last repair that needs to be done. I quite enjoy the dimensions and features of the BZ10, but if it can't brew a decent cup of espresso, then it ain't worth anything at all!

The BZ 10 is a well proven and functional m/c.
If you continue to have problems then you need to look at external factors affecting the m/c performance..
EG..
Power supply
Water supply
Operating techniques
Technician Service skills (fault finding etc)
Etc.