It's hard to tell anything from just looking at it. My suggestions would be to turn on Show Formatting (the ¶ button on the Home ribbon tab under Paragraphs) and also to turn on the Reveal Formatting pane (SHIFT + F1 keys if you're in Windows). On the Reveal Formatting pane, be sure to select the Distinguish Style Sources checkbox at the bottom.
My guess is that one or more of your styles is set to "automatically update," which means when you make a direct (manually applied) formatting change to one area, it affects others. If you switch that off, that may help.
Ultimately, you'd want your heading to be a different style than your body text. If you are creating a bulleted list, as shown below, your list is likely changing to the "List Paragraph" style.
https://preview.redd.it/bpsiuat75byf1.jpeg?width=864&format=pjpg&auto=webp&s=6a7116a028ecf28bcdfdda438b1a2722425e6c60
