each par in a doc has different rulers
some have a default every n mm
need to remove all the tab stops in all the rulers
and then set new tab stops for the whole document
Using the ruler to set tab stops is always a BAD idea.
You can set tab stops globally (not recommended) at Tools → Options → LibreOffice Writer → General.
You can set tab stops manually (not recommended) for a paragraph with a right click → Paragraph → Tabs tab (no pun intended).
Finally, you can set tab stops at the paragraph style level (highly recommended!) by editing the paragraph style in use.
To clean manually set tab stops, select the text and clean direct formatting (Ctrl M).
If you are interested in learning more, I discuss tab stops (among many other topics) on my book about Writer 