I have a 37 pages document (Office 2016, Windows 10), with around 15 Heading 1 (chapters, new page each)

From Chapter 10 on, the heading wording gets a strange behaviour, say, from
10 FINAL QUOTATION
to
10 ............ INAL QUOTATION

Looks like the first letter (F in this case) is substituted by a series of dots, ressembling a TDC style.

The TOC, the headings 2 and 3,... everything else is fine

May I get some hints, please, about what can I check?

thanks