This is not a bug. Rowlock and pagelock are mutually exclusive, and both state to use either row locks or page locks as long as escalation to page locks is not required.
I think that lock escalation can't escalate to page lock. If I'm not mistaken until SQL Server 2008 lock escalation could escalate directly to table lock and since SQL Server 2008 if the table is partitioned, it can escalate to lock a partition in the table (or few partitions if needed).
Adi
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