Just updated to 25.8.4.2 and a new rather strange issue has arisen. When writing in LibreOffice, if I start a paragraph with an indent using TAB (not auto-formatted indents), when I press Enter to start a new paragraph the previous paragraph’s indent disappears. I can go back and add it, but why is it doing this? It didn’t do that before. Is there some auto-formatting setting I need to change/disable?
Menu Tools - AutoCorrect - AutoCorrect Options - Options tab.

Created a bug report: tdf#170547.
Awesome, thanks!
Between then and now I reverted to 25.8.3.2, which seemed to not have this issue. However now whenever I press enter it messes up the positioning of pictures in the previous paragraph (finicky picture placement being one of LibreOffice’s weak points). Doesn’t happen with shift+enter. Any suggestions? I’d really like to be able to keep writing without worrying about messing up stuff earlier in the document!
Because this is a line break, not a paragraph mark. You stay in the paragraph. Don’t try to outsmart Writer by using line breaks. Superficially, you’ll think you’ve found a workaround, but you create potential devastating issues (Writer does not like very long paragraphs).
I would not say it is a weak point. It is very tricky if you don’t use frame styles. But, unfortunately, frame styles are very very difficult to master. It requires a tremendous mind adjustment to understand the underlying principle. But then, your pictures will have a stable and predictable location, no matter how you edit and review your document.
If you want advice on picture positioning, open a separate question and attach a 1- or 2-page sample. Explain clearly the problem.
Note that frame styles, hence pictures too, are extremely sensitive to direct formatting, even in the seemingly innocuous guise of moving a picture with the mouse or resizing it. Such direct formatting ruins completely the layout, invalidates frame styles and is very difficult to remove in the hope to revert to a clean state.
Are you practising styles? They are the only way to achieve stable and predictable layout.