iCloud Calendar only syncs reliably one way: iPhone -> iCloud

Many iCloud Calendar new/recent entries are not being transferred/copied/synced to my iPhone or iPad, but ALL new iPhone Calendar entries will sync to iCloud. I can get various results (having more or less sync) depending on my "Calendar" settings on the iPhone ("Sync" > All Events/Events 6 Months Back/... etc.). But keep in mind that the iCloud Calendar events 'not being synced' are ALL recent and within the last month - so that setting should not be having any effect, and yet it does!

There are no obvious reasons why these iCloud Calendar entries (some created directly on iCloud Calendar, and some on a PC app [emClient] and synced to iCloud with CalDAV) are not being "seen" on my iPhone and/or iPad. And the various events that do not sync are not consistent on the iPhone vs iPad, some appear on one device but not the other! Also, there is no obvious correlation in type of calendar event that fails to sync: some are "All day", some are a certain time, some are multi-day... I have absolutely made sure that I've selected the default "Calendar" to sync and it is checked on both devices(locations). I have also un-checked/re-checked with a full iPhone shut-down in between. I have also completely logged out of iCloud from my iPhone, deleting iCloud data on my iPhone, then restarting my iCloud service -- but nothing fixes this issue.

I have an iPhone 11 Pro Max, running iOS ver 13.6.

If I create an Calendar event on the iPhone or iPad, it ALWAYS syncs to iCloud, but the reverse is not working consistently and causes me to miss appointments that are not showing up on my iPhone. Any help/ideas/(new)suggestions would be appreciated!