So, I am presuming you are referencing the 'message class' from the outlook view, fields. Also that on the un imported messages you see Ipm.note, I am also assuming that in the pst folder when you look at the message class, you see ipm.note.
So, the action of moving from pst to inbox/sub folder removes the .note then it is a strange issue.
Sorry, havn't come across this, and to me it looks like an MS call as opposed to EV, maybe someone else has seen it before.