If I am understanding you... then Yes... this is the behaviour I am seeing...
If you use Add Multi-view Part (MvPartAdd) dialog and the detials pane is open, then you see a list of all the parts along with all their properties... each avaliable part sizes properly shows the ElevToBtmAdj that I've assigned to each... But if you go to Part Filter Tab and manually type in EleToBtmAdj that doesn't match up with a part, that is when you get 'The desired part does not exist in the size needed in the current catalog and the part does not support custom sizing'. Click OK you then get 'Part Size Not Found' dialog with option to Ignore and continue' OK and HEP button.
The 'DecSize' property I also added to the definition (used to sort the parts when scheduled), is also available and can be selected from. The behaviour is the same with this property also. I would rather not have these two properties available to the user ... but if I turn their visiblity off... then I cannot use them as I need for sorting the schedule and in calculations to modify the locationZ elevation. (the part needs to insert at one point, but the LocationZ needs to be modified to show the elevation I want in the schedule.