Ran into this issue aswell. Was able to fix it.
I might be wrong but, it seems that the the email determines locale of the application or account. (email server location) My wife had the version v2.2.3_763 (EU) and I had version v2.2.3_763 (US). You can check this by going in the menu in the Eufy Security app and clicking about.
My wife used a email address with a custom domain, hosted in the Netherlands, I just simply used my gmail account. Which in turn gave us different versions of the app. I solved it by creating a account that uses the same domain as my wife’s email address. After that she received notification when I sent the invite.
TL;DR
Make sure you and the people you want to share it with have the same version of the app (EU/US). If this is the case, make sure you have the same email domains.