The 50-to-30 pigtail only connects to one leg of the 50A socket. Think about it: your 30A RV has only one AC bus and if the pigtail connected to both 50A legs, it would short those two out-of-phase legs.
That said, given good connections, there should be no difference in the behaviour drawing a maximum of 30A from a 50A-capable leg through the adapter or directly from the 30A-capable 30A socket.
But the fact that it was your own 30A main breaker that tripped is probably the clue. Note that above I said 'good connections'... Its possible that somewhere in the connection chain through the 50A socket, there was a high resistance connection that caused low voltage, which would cause your A/C to draw a higher current, along with everything else, than it would have with full voltage and thus it went from 'just under' current to 'over current' and tripped your main.
This would be unusual because often the 50A connection is in better shape than the 30A but maybe the power post is old and they had just fixed the 30A so that it was in better shape.
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam