I am using DAVx5 Version 2.5-ose installed from F-Droid.
I have connected to two CardDAV accounts on my Nextcloud server. When creating a new contact on my Android (version 8.0.0) phone using the contact app I can choose to save to these accounts and they are synchronized. However, several apps, for example K-9 Mail, do not seem to have access to these accounts. In K-9 mail I can only see my phone contacts. In another unrelated business card scanning app, the only accounts available to save contact details are my local phone and google account, the DAVx accounts aren’t an option. The phone is a Sony xperia F5321.