Magento 2: theme translation does not work

Question

Magento 2.2.9

I have a third party module (vendor/mirasvit/module-search-autocomplete). It comes with several translatable strings (i18n/en_US.csv).

I want to translate the string Show all %1 results → in my theme folder (theme/i18n/de_DE.csv): "Show all %1 results →","Zeige alle %1 Ergebnisse →"

That file already contains a lot of working translations (mainly from magento core) but no matter what I do, this string will not be translated. I cleaned caches (magento and browser), I deleted everything in pub/static/frontend, setup di compile... nothing helps.

Just for testing purposes I also modified the translatable string in the modules core. Just to make sure it is the right one. Nothing.

It only works when I put a de_DE.csv file into the modules i18n folder. But that is not what I really want to do for obvious reasons.

How? What? Why? It does not make any sense to me.