Outlook REST API is returning 404 for event in message - outlook-restapi

Question

I have the following case:
I use the Outlook REST API (without any library) to visualize the users' emails - including Event invites. First, I make a call to get only the message properties and if the message has MeetingMessageType value, then another request is sent to:
https://outlook.office.com/api/v2.0/me/messages/<messageId>?$select=Microsoft.OutlookServices.EventMessage/MeetingMessageType&$expand=Microsoft.OutlookServices.EventMessage/event($select=Id,SeriesMasterId,iCalUID,Type,CreatedDateTime,LastModifiedDateTime,WebLink,Calendar,Start,End,IsAllDay,IsCancelled,Organizer,Attendees,Location,Subject,ResponseStatus,OnlineMeetingUrl,Recurrence,ResponseRequested)
to get the Event information and provide the customer with the option to respond to the invite.
The problem:
Since yesterday one of our clients reported that he can't see the event information. It turns out that the request for getting this information is returning 404.
The response body:
{
"error": {
"code": "ErrorItemNotFound",
"message": "The specified object was not found in the store., The process failed to get the correct properties."
}
}
The event is shown in the Outlook desktop mail client.
The client also shared that they have enabled ATP (advance threat protection) at his company. Not sure if this can have anything to do with the problem.
I see that other clients also have this problem.
Update:
I asked the user to try the following requests from the Outlook sandbox:
https://outlook.office.com/api/v2.0/me/messages/<messageId>
This request returns the EventMessage with "MeetingMessageType": "MeetingRequest".
Then when he tried accessing the Event:
https://outlook.office.com/api/v2.0/me/messages/<messageId>?$select=Microsoft.OutlookServices.EventMessage%2FMeetingMessageType&$expand=Microsoft.OutlookServices.EventMessage%2Fevent
The Outlook REST API returns 404.
For me, the same formatted requests work, so I can't really understand what is going on.
Please, any information will be well appreciated!