To replicate: 1) Find or create an item with itemnotes_nonpublic populated. 2) Go to the 'Normal' tab (catalogue/detail.pl) for that item. 3) Note that 'Non-public notes' column shows the the non-public note. 4) Click the Edit button on the right side of the holdings tab (.../cataloguing/additem.pl?op=edititem&biblionumber=XXX&itemnumber=YYY#edititem) 5) Note that 'x - Non-public note' is empty. I've verified this in 18.05.10
Also verified under 18.11.03
This does not seem to happen in Master.
Nevermind, itemnotes.nonpublic wasn't mapped to 952$x in itemnotes.nonpublic for the sites that I was testing.