According to Microsoft a bug exists in Cumulative Update 1 for Exchange Server 2019 and Cumulative Update 12 for Exchange Server 2016 where users can no longer accept Share Calendars through an email invitation.
Microsoft has supplied the following workarounds. Use one of these until a final resolution is available.
> Open the invitation from the Notifications pane in Outlook on the web.
> Add the shared calendar manually in Outlook on the web.
> Open the invitation in Outlook, and then add the shared calendar.