To be honest: not really. But maybe the developer can give some insight?
I have “German Formal” as default language in WPML and “English” as a second language. When I define “German” as fallback, I see German strings on English pages.
For example my English product shows (using WooCommerce) “In den Warenkorb”, “Zus?tzliche Informationen” instead of “Add to cart” and “Additional information”.
Maybe this can help (original title of support ticket was “Fallback of the fallback”): https://make.www.ads-software.com/core/2017/05/20/preferred-languages-the-prototype/