"Call to undefined function OC\\Security\\idn_to_utf8() in file
'/usr/share/webapps/nextcloud/lib/private/Security/RemoteHostValidator.php'
line 38"

Hah I should have read your question properly. idn_to_utf8 function *IS* part of intl, yes, but look at the namespace it's moaning about.

This means, either, the function is overridden in that namespace and the file it resides in isn't loaded, OR, the "real" function was imported into that namespace, but again in a file that isn't loaded.

To me this looks more like a bug in the nextcloud rather than PHP

I did see a ticket being opened on nextcloud talking about this same thing, presume this was yourself?