It's not a bug, it works with both Content Controls and Legacy Form Fields. It's the basis by which all Word formatting works. Unprotect the form, select the field (all of it, not merely that part within the field) and apply whatever formatting you require. Re-protect the form.

If the 'field' in question is a Cross Reference field, taking input from another part of trhe document, then use a CHARFORMAT switch on the field to ensure it retains the applied formatting.

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Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com