No this isn’t random. When you are at the end of a paragraph, be it empty, when you hit
Return, you open a new paragraph with all the attributes in effect at that time. There is no implicit “reset” when starting a new paragraph. This allows formatting continuity.
Consequently, if bold, bullet or 64pt were active, your new paragraph starts in bold, bullet, 64pt.
The only way to avoid unexpected behaviour is to format your document exclusively with styles, but you get continuity in character styles. Paragraph styles may automatically switch to another style if correctly configured.
However, some attributes are somewhat outside this ideal scheme. Bullet/numbering are among those. Depending on how you bulleted/numbered the current paragraph, bullet/number may be “sticky” when you open a new paragraph. This frequently happen if the bullet/number was added with toolbar buttons (direct formatting) instead of being brought in by the paragraph style.
If you really want a very predictable formatting, learn how to use styles (but, to be honest, there are still areas where the behaviour challenges your mind and patience – again bullet/numbering are frequent contenders).
Direct formatting looks “intuitive” to beginners. It’s really great for experimenting but to be comfortable with it requires you to be an expert, otherwise you’ll get out of your nerves with some surprising yet perfectly logical behaviours (once you have a precise mind model of what’s going on). So to be on the safe side, use styles and avoid as much as you can direct formatting.
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