Please see the question on this page: https://ask.libreoffice.org/en/question/188730/entering-a-changing-the-font-on-that-line/?comment=188896#post-id-188896 If you enter the unicode character 2b1b (⬛) in some fonts, and then type, the font will change. Adding line breaks seems to make it show up more. I've experienced it changing seemingly randomly from a serif font to a sans-serif font back and fourth while typing. The name of the font will not change, but the appearance will. If you review the comments in the question, two users have reproduced the bug.
Created attachment 159135 [details] reproduction of font bug As you can see, by typing and adding line breaks the font changes randomly after entering the square character with the font set to Font Awesome. It also occurs with some other fonts. The text is all highlighted to show that the font is all the same, otherwise it would display a blank font type in the font selection menu.
As noted in the Ask question, this is the os/DE managed font fall back mechanism when you specify a Unicode point that does not have coverage of the codepoint in the font specified for the paragraph. You may avoid any vagaries in font fallback controlling the font used by using the Special Character dialog and selecting a font with coverage of the codepoint being inserted. Selection of font and glyph using the Special Character dialog will also embed the font as direct formatting into the document, avoiding font fall back mechanism that can result in misformatting. And unfortunately as noted, LibreOffice does not do a very through job of alerting when individual glyphs for codepoints are being replaced by fallback, showing font name in italice only when entire fonts are unavailable. Think there are open enhancements for both. Otherwise really is not a LibreOffice bug.