iPad and iPhone show one delivered iMessage but not on the other. Is message delivered?
I've experienced this when the sending device was the iPhone, and I knew the recipient was on holiday and not online - the iPhone did not show delivered, but the iPad did. Eventually, when the recipient came back online days later, the iMessage showed delivered on the iPhone too.

This is a likely an iMessage bug/anomaly (which I wish Apple would address!) I would take the true delivery notification to be the device from where the message was originally sent.