User informed when he tries to modify a particular series of recurring meeting the changes reverts back next day.

Hi,

Please make sure this user's Office is up-to-date, I've seen updates that fix the similar issues for previous Office.

I've also seen this issue happens to some users who use the calendar on phone, is the user or his delegate using this calendar on phone?

Let this user modify the recurring meeting on OWA, check if the problem persists.

Regards,

Melon Chen
TechNet Community Su

Thanks for reply. Will check and update u