Formulas in Custom Properies not "sticking"

I have a particular custom property field which I populate with a formula that combines the elements of other custom properties (in the attached image it is the "Material Specification" property).

This formula seems to work fine for a while, but occasionally (actually very often) I will come back into the properties after a rebuild, or tweaking something and find that SW has overwritten parts of my formla with other stuff.

Any ideas why, or how to fix?

Cheers, Stew.