Outlook: Tasks of Exchange account not showing in To-Do List

Good Day,

I got forwarded to this community and hope to find some help here.

I'm having an issue with a single user (I'm an administrator in our organisation) where in Outlook, the tasks listed in the user's own Exchange account don't seem to show up in the To-Do List (in Outlook).

The ones the user created within their private Hotmail account which they've added in Outlook appear without any issues. Affected seem to be only the ones the user created within the Exchange account.

In the filter settings of the To-Do List (View > View Settings > Filter), there does not seem to be any filter active that may be filtering out the tasks from the Exchange account.

We are using Exchange On-Prem and standalone Office (Office 2019, Office 2021, etc.), although the Office version doesn't seem to make a difference, as adding the affected user's Exchange account to my Office/Microsoft 365 installation seems to have the same effect.

In Microsoft Admin Center, the user has the respective license (A1) to use To-Do (Plan 2) and the To-Do function is active for the user.

I don't know where else to look and some help would be greatly appreciated.

Thanks and Best Regards,
Lukas