When a block flow element is inlinified, it becomes inline flow-root (i.e. inline-block) in order to preserve the block container.

Shouldn't the same happen for block ruby? If you request your ruby container to be wrapped inside a principal block container (e.g. because you want to use width or height), shouldn't the block container be preserved (in the form of as inline-block) in case the element is unexpectedly inlinified by some ancestor?

I think so. There should be a ruby-root inner display type and, just like flow becomes flow-root during inlinification, ruby should become ruby-root.