As the facts in this case are yet to be revealed I cannot comment specifically on what happened to the A380 however, what I can say regarding any type of GNSS approach (RNAV-Z or RNP-AR) is that you must not arm the approach pushbutton unless the ATC assigned altitude and the IAF (or suitable Direct to waypoint) altitude are the same, otherwise the FMGC switches from NAV to FINAL APP mode and therefore will drop down to the NEXT altitude constraint as per the FMGC constraints, thereby bypassing the relevant safe altitude even if it is still selected in the FCU.
This was why, some time back Airservices changed the terminology to "when established cleared Runway xx x approach" that is to say:established on the approach itself and not simply tracking to intercept a waypoint on the approach.
In simple terms, selecting APP mode early on the FCU has the same effect as (spuriously) being in LOC capture but you're only tracking to the IAF and not yet runway aligned, therefore you might get G/S capture on base for example.
In FINAL APP mode the aircraft likes the track so it descends, it's not smart enough to see that a direct to is not necessarily on the approach itself.
If the A380 did what I'm referring to (yet to be determined) then it's not the first time it's occurred on YMML 34.