I believe I have the same problem as Writer doesn't save page styles - #2 by Hrbrgr,
in writer 7.6.1.2 (X86_64). But I am saving as .odt, so the answer there does not solve my problem.
A major (but not the only) use case for me in MS Word / LibreOffice Writer is to combine photographs into a single pdf (to send them as documents, and to reduce file size), for which I want to reduce margins to 0 on all sides. In MS Word, if I create a new layout , it remembers that layout and it will be suggested to me the next time I want to select a different layout. In LibreOffice, I have tried creating a new page style (in page style pane > rightclick > new, create a new style with margins=0), and creating a template, and both I was not able to find again when opening a new document.
One suspicion is that margins are not handled by “page style”, but then that settings for margins should not be available when defining a new “page style”. Also, it does not solve page styles not being remembered.