Can't delete a UOM Class conversion

Trying to delete a UOM conversion - which I don’t think is being used anywhere - but get an error saying I can only make it Inactive.

I’m not trying to delete the Class or the UOM itself, but rather the conversion of this UOM for the particular Class. The UOM Class is LIQVOL (for liquid volume UOMs), and the UOM I’m trying to remove is “EA” (which is valid for other Classes, like COUNT)

When I try to make it Inactive I get

A query of the Part table show no parts using this UOM Class / UOM Combo.

I was able to delete other LIQVOL conversions (like BX) that are in use elsewhere