Problem with email recipients being modified after update to version 2306

A colleague who uses Microsoft 365 for Business received the update to version 2306 sometime on June 26. He uses Outlook for desktop and experienced an issue immediately after this update where SOME but not all recipient addresses were incorrectly modified when messages were sent to multiple recipients. The recipient addresses were manually typed into the To: and CC: fields in the form mailto:email address removed for privacy reasons however the addresses in the To: field were changed when the message was sent to the form mailto:email address removed for privacy reasons and these generated non-delivery reports. My colleague uses the auto-complete list instead of contacts. I have checked his NK2 list and all of the addresses are correctly listed there. There are no instances where the addresses show as mailto:email address removed for privacy reasons, however some other process must have changed the address to this incorrect format. 

We suspect this is related to the version 2306 update. Checking his Outlook version today shows it to be version 2305 rather than 2306 which it was yesterday (June 27). The recipients are executives at a major company he does legal work for and he would prefer not to bother them with any testing until we have reasonable evidence that this issue will not recur, so as not to waste their time.

Is this a known issue and would the change from version 2306 to 2305 have corrected it? Is there any other potential resolution to try? Before sending future messages to these recipients (an activity that needs to be done almost daily or several times each day) he wants to be certain that this issue will not happen again.