We clear the OplogBuffer in _resetState_inlock: https://github.com/mongodb/mongo/blob/8c5d01498d1dfa8eef4292db4d32e6e6465a1580/src/mongo/db/repl/data_replicator.cpp#L352

When the call to this function got moved from the end of initial sync to the beginning, we no longer cleared the OplogBuffer at the end of initial sync.