Matrix blocks can be initially propagated with the wrong site’s content

@brandonkelly Thanks for the fix! Couldn't see any loss now, but it seems things can go wrong when done in a wrong sequence.

Example (same site settings from #6441):

Entry exists in berlin_de and berlin_en with translated content

a) select berlin_de, add new site munich_de => matrix blocks are propagated, field correctly has the german content
b) select berlin_en, add new site munich_de => matrix blocks are propagated, but field has english content. (so have other translateble fields too)

Looks like content is always propagated from the currently selected site and the fields translation settings are not respected.

Shoudn't be a problem in 'one-dimensional' site settings (german,english,italian/translated by language OR berlin,munich,kleinfinstenich/not translated), but in 'multi-dimensional' settings, unless you enforce a very strict workflow (first add sites, then add content)

(add translate by site group to the mix :-) )

Originally posted by @wsydney76 in https://github.com/craftcms/cms/issues/6441#issuecomment-663844084

Reproduce the two site-creation sequences using the same site settings referenced from #6441, comparing the initial Matrix block content and other translatable fields. No source file or test is named; done means the reported sequence no longer initializes the new site's fields from the wrong selected site's content.