For the record: I’m not Victron staff but an installer just like you.
So far I’ve never encountered the troubles you’re reporting.
Last time I noticed completely bogus grid usage values, the L1-L2-L3 current transformers positions of the VM-3P75CT meter didn’t match those of the voltage sense wires.
But yes, there seems to be a bug in the v3.60~60 beta version in regards to the VM-3P75CT meter.
See this thread.
If that matches your problem, you are probably hitting that bug.