I selected kmail above, but this could be a korganizer bug.  I'm really not sure.  I have reproduced it on Deepin, Ubuntu 19.10, 19.04, and 18.10, Kubuntu, ArcoLinux, Manjaro, Solus, and maybe a couple of others.  I've distro hopped so much due to this bug I can't really remember.

Kontact starts with an empty identity, meaning, no value in Name, Organization, Email Address, or Email aliases.  If I connect to my Office 365 account and retrieve all my mail and calendar items without modifying this identity, everything is fine, with the obvious exceptions caused by not having an identity with data in it (sending mail is pain because it doesn't know who the mail is from, etc).

If I then fill in *any* data in my identity, I get several hundred message boxes stating, "Error while trying to delete calendar item.  Error was: No items found", and most of my calendar items disappear.  All of the ones that disappear are recurring, the ones that stay on the calendar are not, but that doesn't mean much because almost all of my calendar items *are* recurring.

If I fill in the identity information first before creating the Office 365 account, I don't get the popups, but I never get the recurring calendar items in the first place.

I have verified that the data is still present via the akonadi console, and the missing items can be recovered by renaming the identity that caused the issue, creating a new one with empty fields, and clicking cancel in the "Edit Identity" window, then deleting the original identity.

This bug effectively makes KMail a mail reader, since sending mail is too much of a hassle with no identities defined.

Also, creating the accounts and identities in kmail as opposed to kontact has the same effect, sans the message boxes.

STEPS TO REPRODUCE
1. Start Kontact with the default identity only.
2. Create a new receiver for an Office 365 account.
3. Verify that calendar items are present, and at least some are recurring.
4. Click Settings->Configure Kmail, select an identity and click Modify.  Enter an email address.  Click Ok.
5. Close about a billion popups
6. Verify that calendar items are missing.

OBSERVED RESULT
Recurring calendar items are missing after filling out identity info.

EXPECTED RESULT
Calendar items should remain unchanged.


SOFTWARE/OS VERSIONS
Windows: 
None tested
macOS: 
None Tested
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.66.0
Qt Version: 5.14.0

ADDITIONAL INFORMATION