What version of Fava are you using? I just tried setting option "account_current_earnings" "Equity:Gewinnvortrag" in my beancount file, using Fava 1.30.1, and beancount 3.1.0, and it appears to work as intended. I get Equity:Gewinnvortrag under fava, instead of Equity:Current.
What does it show under the "options" menu/link in Fava? It should show the current fava, as well as beancount options, and what files are being loaded. Perhaps you're not including the option declaration in the proper / current beancount file?