Have you tried using the "Legt Text aus Zwischenablage fest" (in english: set text to clipboard) function? Set it to the required "whole" string ("=Kalenderwoche(D1;21") and instead of typing it, then, use the function "Ruft Text aus Zwischenablage ab" (in english: pase text from clipboard) to paste it in the correct field. He shouldnt ask you for any formating then.
(so instead of writing the whole thing out, set the clipboard (zwischenablage) to the required text and then paste it in the field)
Also id assume you already do that, but make sure the flow writes the values here instead of here:

Excel likes to be stupid with formatting. If you're lucky that will solve it, if it doesnt, please get back to me and i'll try to help.