Using an answer, but this is not a solution; I break the rule because of limited space in a comment.
Your sample file is plagued with direct formatting. Instead of using a custom character style, you tagged words as Polish with Format>Character and forcing language to {pl}.
- Direct formatting explains the
style-name="T999" in the XML. T999 is used for character style. When you look at the style dictionary, you see that this style id has no name (this is an indication of direct formatting).
- Language name
{pl} is not the correct designation. It should have been chosen from the drop down menu. But it does not seem to make a difference because after correction, the "T2" character style still shows fo:language="pl".
Not all Polish names are tagged with the character style. It does not relate to the problem unless even in this case they look different. Check all your markings in the sample file.
My Linux box has no Source Serif Pro. Consequently, it is substituted just like Microsoft Sans Serif but I could find no mention of it while scanning the text. However, Microsoft Sans Serif is set in paragraph style Default Style. This means this font may come in the way if something is not configured correctly in the paragraph styles because all styles inherit from Default Style if an attribute is not overridden.
Not related to the problem at hand: do you know you can generate your TOC automatically if you style your headings with Heading n? Questions on this site provide tricks to have chapters numbered “Chapter 99” and restart numbering with “Appendix 99”, both sequences ending in the same TOC. Anyway, you didn’t configure paragraph indents which would solve some editing issues. Also you inconsistently use line breaks (Shift+Enter) and paragraph breaks (Enter) to handle multi-line headings.
I notice you have a plethora of Body text (99) and Heading # 9 (9). It might be worth thinking over the design of your styles. There is no reason to define one style per paragraph. A style is usually meant for a family of paragraphs belonging to the same semantic field. I guess the various PageStyle9 result from the necessity to have different header in different chapters. This can be solved with field insertion, thus a single page style handles several chapters.
There are also several sections. What are they for? Sections are legitimate when you want to temporarily change the number of columns with regard to the current page style. Other than that, they are a nuisance. I see them frequently in documents coming from M$ Word realm. Sections in Writer are not the same as in Word. Sections in Word are rather close to Writer page styles. Use of the same word for different concept is unfortunate.
I could not find the origin of the problem. When you put the cursor in a Polish word, does the font drop down menu in the toolbar still displays Source Serif Pro?
If you want more assistance, post a screenshot of the difference in font with an extensive description of the phenomenon (because I have not the fonts you chose).
Also, I’ve added the version info to the original post.