My guess is that you are using Paragraph Composer in InDesign, which recomposes the whole paragraph based on best-overall spacing (not just one line at a time).

Sorry, you cannot have an anchored object thread to a new page. Anchored objects cannot be threaded and cannot span multiple pages.

This might help you with the problem you’re having with the anchored objects moving unpredictably:
https://creativepro.com/creating-dynamic-layouts-objects-move-text-added.php

I also recommend you learn about automatic paragraph numbering and how they can work across frames:
https://creativepro.com/setting-up-numbered-lists-that-jump-frames-in-cs3.php

Yeah, this has happened to me a lot, just on certain documents… and usually on pages with tables. I don’t know why. Next time it happens, try pressing Command-Option-/ (slash) or Ctrl-Alt-/ which forces the screen to redraw.

This has been a conundrum for many years. Here’s a post I wrote about it:
https://creativepro.com/removing-space-along-left-edge.php
(be sure to read the comments for a script… though I haven’t tested it in a long time.)