I cannot tell from a screenshot but I would guess that you started with direct formatting the text and after the first heading, the body of the text changed from Default Paragraph Style to Body Text.
Default Paragraph Style is not intended to be used in documents, rather it is there to make global changes. All styles inherit from it, either directly, or through another style. This is where you would set you Minecraft font face. You can see the inheritance in the image below. If a lower style is changed, then those attributes that are changed have their inheritance cut and they will no longer update to reflect changes in in Default Paragraph Style. To restore inheritance of an attribute in a child style, select the tab in which the attribute is set and then click the button Reset to parent
Body Text is intended for the body of the document. To alter the settings for it, right click the paragraph style in the Paragraphs pane of Styles in the Sidebar and select Edit style. Change the settings for Spacing here. Body Text can be applied by clicking in the paragraph and pressing Ctrl+0
Headings inherit their font face from Heading style in the Sidebar, so this is where you set the font face to Minecraft for all headings. Heading 1 can be applied with Ctrl+1, Heading 2 by Ctrl+2, etc.
You probably have direct formatting in your document so having made the settings above, select a number of paragraphs and click Format > Clear direct formatting (Ctrl+M). See if it fixed your problem and if you need to make any further changes to the styles.
There is more on styles in the Writer Guide, download from English documentation | LibreOffice Documentation - LibreOffice User Guides