This is listed as a known limitation
Migrated meeting invitations that are unread in Notes are not displayed in the OWA calendar until after they are viewed post migration. (21865)
If the user strictly uses the OWA client, the calendar will appear as tentative on the outlook calendar once they are read from the inbox in Exchange.
If the user is using Outlook client, the behavior is the same, unless they have "Automatically process meeting requests and responses to meeting requests and polls" selected, under File/Options/Mail/Tracking. If this choice is selected the meeting will eventually show on the calendar as tentative as the Outlook client processes them. They will not be there immediately after migration if viewed after the migration. Outlook needs time to process them but they will eventually show on the calendar.