I am experiencing the same problem here. Nextcloud 18 and iOS (iPad Pro with the latest iOS (iPadOS 13.4.1)). As I only use tasks at the moment (calendars and contacts are still empty at the moment) I can only talk about tasks.
Tasks worked for 1-2 weeks after setting everything up. Well, I had to sync by hand (by swiping down in the Apple Reminders app), but that was not a problem at all. But after 1-2 weeks it suddenly stopped working.
And now the interesting part: New tasks are still synced at once. But existing tasks are not updated (not getting the “completed” flag or a new reminder date/time).
When I check the log I sometimes (!) see the following:
{"reqId":"XsUSllhjWpIAABNEerkAAABB","level":0,"time":"2020-05-20T11:20:55+00:00","remoteAddr":"2003:c5:9f0a:e900:60f0:4e03:c64e:fb08","user":"martin@sebald.com","app":"webdav","method":"PROPFIND","url":"/remote.php/dav/calendars/martin%40sebald.com/","message":{"Exception":"Sabre\\DAV\\Exception\\NotAuthenticated","Message":"No public access to this resource., No 'Authorization: Bearer' header found. Either the client didn't send one, or the server is mis-configured, No 'Authorization: Basic' header found. Either the client didn't send one, or the server is misconfigured","Code":0,"Trace":[{"function":"beforeMethod","class":"Sabre\\DAV\\Auth\\Plugin","type":"->","args":[{"absoluteUrl":"https://domain.tld/remote.php/dav/calendars/martin%40sebald.com/","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]},{"file":"/var/www/clients/client2/web258/web/3rdparty/sabre/event/lib/EventEmitterTrait.php","line":105,"function":"call_user_func_array","args":[[{"autoRequireLogin":true,"__class__":"Sabre\\DAV\\Auth\\Plugin"},"beforeMethod"],[{"absoluteUrl":"https://domain.tld/remote.php/dav/calendars/martin%40sebald.com/","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]]},{"file":"/var/www/clients/client2/web258/web/3rdparty/sabre/dav/lib/DAV/Server.php","line":466,"function":"emit","class":"Sabre\\Event\\EventEmitter","type":"->","args":["beforeMethod",[{"absoluteUrl":"https://domain.tld/remote.php/dav/calendars/martin%40sebald.com/","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]]},{"file":"/var/www/clients/client2/web258/web/3rdparty/sabre/dav/lib/DAV/Server.php","line":254,"function":"invokeMethod","class":"Sabre\\DAV\\Server","type":"->","args":[{"absoluteUrl":"https://domain.tld/remote.php/dav/calendars/martin%40sebald.com/","__class__":"Sabre\\HTTP\\Request"},{"__class__":"Sabre\\HTTP\\Response"}]},{"file":"/var/www/clients/client2/web258/web/apps/dav/lib/Server.php","line":319,"function":"exec","class":"Sabre\\DAV\\Server","type":"->","args":[]},{"file":"/var/www/clients/client2/web258/web/apps/dav/appinfo/v2/remote.php","line":35,"function":"exec","class":"OCA\\DAV\\Server","type":"->","args":[]},{"file":"/var/www/clients/client2/web258/web/remote.php","line":165,"args":["/var/www/clients/client2/web258/web/apps/dav/appinfo/v2/remote.php"],"function":"require_once"}],"File":"/var/www/clients/client2/web258/web/3rdparty/sabre/dav/lib/DAV/Auth/Plugin.php","Line":168,"CustomMessage":"--"},"userAgent":"iOS/13.4.1 (17E262) remindd/1.0","version":"18.0.4.2"}
Not every time and when I saw it, new tasks were synced.
I also tried to set up app passwords, but it did not make any difference.
At the moment I have no idea where elso to look.
Regards,
Martin