Does anyone have problems getting all contacts (or their changes) synchronized between Fastmail and iOS and macOS devices (being on Tahoe / 26.X)?

I noticed the other day that out of my 108 contacts in Fastmail, only 90-ish showed up on my mac, only 50 on iOS and changes one way or another did not propagate to the other end(s).

Mind you, under Fastmail's settings > Migration > Apps & Setup guides > Connected apps I can see that both app specific credentials (so the one for iOS and the other for macOS) have continuously been used successfully to access Fastmail.

What I tried yesterday then is to remove the Fastmail device management profiles, delete the app passwords for them.. and re-do the automatic/profile based approach on both devices ... just to yield the exact same problem (but with a different, but still not complete set of contacts). Same thing thereafter when doing it manually by adding CardDav server/credentials (and an additional set of app specific passwords etc). Yet another amount of contacts synchronized.. but not all.

Sooo not sure what's going on. I got no special groups, shared contacts or whatnot.. just the default set(up) where I've all my contacts in in Fastmail.. and yet I'd very much like to be able to see who's calling my on the phone etc -.-

Does anyone know where one might be able to see what's going on and/or has a solution for this?

Synology is using Radicale, an Open Source CalDAV and CardDAV server.
I haven't looked which version they're using but I know that it's limited hence compatible with very few clients like Thunderbird and absolutely not compatible with MacOS contacts.
Synology support has claimed that the problem is coming from Apple. Actually this is entirely false. And here's why:

"CalDAV and CardDAV are not perfect protocols. We think that their main problem is their complexity, that is why we decided not to implement the whole standard but just enough to understand some of its client-side implementations." Source: https://radicale.org/v3.html

It seems that Synology has elected to privilege its "office"-like suite and reduce compatibility with standard operating systems. The burden falls then on their loyal customer-base. For how long is something to be seen.

I host my own instance of Nextcloud that works perfectly save for syncing contacts with any of my macOS devices. iOS syncs without issue.

Im using NC 19 RC2 at the moment, but this has been persistent for the last several versions Ive been on.

There are no errors reported on the macOS side, and Im certain the password and username are correct. When intentionally using the wrong password, the authentication challenge tells me so. When using the right combination the challenge succeeds.

The logs within the NC UI aren't helpful either, as I don't see anything relating to trying to log in (successful or otherwise). Where are the filesystem level NC logs located? Ive followed https://docs.nextcloud.com/server/latest/user_manual/pim/sync_osx.html but there's not much troubleshooting detail available. Why would iOS work and macOS not when syncing against the same NC instance and same user?

Anyone else having trouble getting contacts to sync with macOS? What other diagnostic detail can I provide?