Apple Calendar will pull Office 365 calendar events, but will not push added events from the phone to the server

TLDR: Wife's Office 365 account on iOS devices using native apps can only pull calendar events from the server and not push new events.

I have run into an issue with my wife's devices. This happens across her iOS devices (her previous iPhone 14 pro and her new iPhone 16 pro) and her iPad. Basically, the issue is with office 365 calendars using the native Apple Calendars app. Her account can pull the events down to her phone from the server, however when adding new events they are never pushed up to the server to be synced. This is with a shared calendar.

I have done the following testing:

• Added her account to my iPhone using the Apple Calendar app and the problem existed there
• Added her account to my outlook app on my iPhone and this was able to work
• This tells me that there's an issue with how her account is syncing with apple.
• She's unable to use the Outlook app on her iPhone because it is currently used for work.

Has anyone else come across this?