A family of Microsoft word processing software products for creating web, email, and print documents.
I'm not able to reproduce the behavior you saw, but I can suggest a way to prevent the problem.
Right-click the property in the document and choose Toggle Field Codes. You'll probably see
{ DOCPROPERTY Customer \*MERGEFORMAT }
Change the word MERGEFORMAT to CHARFORMAT. Select at least the first character in DOCPROPERTY and apply the formatting you want the result to have. Then update the field.
Read this page for explanation of the field formatting switches.
Jay Freedman MS Word MVP