PTP450 -> PTP450i

We quite frequently use a PTP450i master to talk to a PTP450 slave and it has worked fine.  Done mostly when we do not have a 48 volt source on slave site yet and or limited space.  The other day due to a planned future update I tried to link a PTP450i slave to a PTP450 master.  They would link but not pass traffic.  I tried 15.0.3 and 15.1 on them.  Is there a reason for this?

This is not a supported configuration.

But the PTP450i to PTP450 non-i is?  It works anyway and works well.