I can see no advantage to separating the gen-set earth from the building earth either, in fact I can see several problems with such an arrangement.
I'm thinking along the lines of switching transients as that 4-pole transfer switch is operated. I've run across RCD-tripping problems on just a single-phase system where there was a little earth leakage (from heating elements) and a double-pole isolator had the line side closing a fraction of second before the neutral.
The varying loads on the system at different times could account for why it trips on some changeovers and not others. It could also be dependent upon the point in the A.C. cycle at which the contacts close.
