https://drafts.csswg.org/css-ruby-1/#valdef-ruby-align-space-between

I think it would be useful to have some text, and perhaps another illustration, showing what would happen if you have more than one word in narrow-cell annotations. For space-between, my assumption is that if you have a two word annotation in Latin script over a longer base, the left edge of the word on the left would be flush with the left edge of the base, and the rightmost character of the other word would be flush with the right edge of the longer item.

The question is what happens between the two words. That could leave a good sized gap in the middle, which is what Firefox does, or cause the browser to stretch the inter-character spacing in order to reduce the gap between the words, which is what Edge does. There is a test for this at:
https://www.w3.org/International/tests/repo/run?base=css-ruby-1&batch=css-ruby&test=ruby-align-property/ruby-align-space-between-104.html

Are either of those correct, or is just one appropriate, or is it specifically undefined? Either way, i think the spec should say something.