iPad and iPhone show one delivered iMessage but not on the other. Is message delivered? Sent message from iPad or iPhone and message does not say delivered. But when I look at other device it shows delivered. Is message delivered? Example message sent from iPhone no delivered status. Look at same message on IPad and shows delivered status.
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.