This is due to a conflict with prettier and dprint formatters, with the default vertical side separator for reveal of 4 - chars. Plain markdown doesn't render \n---v\n as a <hr/> and not sure it's worth it to add some workaround to get some mdbook preprocessr to fix it.

Both formatters correct \n----\n to \n---\n , so vertical slides are clobbered if you change them. We want vertical slides, so need a workaround...

One hack could be simply wrapping/prefixing every vertical slide separator in a for matter ignore block and move back to the default separator, like:

<!-- prettier-ignore -->
----

This is easy to do in bulk with a global find&replace, but hard to enforce in writing slide and to lint for. So not ideal, and I would bet lead to at least some amont of frustration and manual review to ensure the formatter doesn't fight the content creators here.