iPhone iOS12.4.1 stopped syncing Calendar to El Capitan

[tuckfoot]

Hi --

Calendar used to sync perfectly between MBPro running El Cap, and iPhone XR, currently in iOS12.4.1

Not sure when it stopped syncing, but it's not working in either direction. MBP entries don't go to iPhone, and vice versa.
Driving me a bit nuts.

Restarted everything. Logged out and back in to iCloud. Both devices say they are seeing iCloud, but sync remains broken.

The only clue is that very old Calendar entries are showing up in both iPhone and MBP.

I'm clearing overlooking something obvious. Any suggestions would be appreciated.

Thanks.

 

[tuckfoot]

Update -- Contacts (address book) sync works fine, but Calendar does not.

 

[posguy99]

iCloud is the medium. Do events from either show up in iCloud?

 

[tuckfoot]

No. Some appear in iCloud, but with a huge time delay (like days). New events entered on MBP do not appear in iCloud.com