Hi EJ,
Thanks for clarification.
I have tried cancelling a reoccurring meeting event by canceling the single occurrence in OWA (Outlook Web App). However, I have not encountered the similar situation. 
Based on the scenario, I suggest you check the following points:
1. Whether the issue happens when choosing to cancel series and creating a new weekly event. To isolate the Outlook client, please close the Outlook client and just do this test in OWA.
2. If the issue persists in OWA, that is to say, the issue can be reproduced anytime, then please let us know. We will collect your tenant information for further investigation.
Regards,
April Zhou