EWS GetFolder bug on Office 365 - calendar

Question

I have an office 365 account with the following accounts :
One main user: mainuser#company.onmicrosoft.com
Another user: someuser#company.onmicrosoft.com
someuser#company.onmicrosoft.com has shared his calendar with mainuser#company.onmicrosoft.com
Then I execute the following EWS operation (through mainuser#company.onmicrosoft.com) to get the calendar permissions set for someuser#company.onmicrosoft.com
<?xml version="1.0"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages">
<soap:Header>
<t:RequestServerVersion Version="Exchange2010"/>
</soap:Header>
<soap:Body>
<GetFolder xmlns="http://schemas.microsoft.com/exchange/services/2006/messages">
<FolderShape>
<t:BaseShape>Default</t:BaseShape>
<t:AdditionalProperties>
<t:FieldURI FieldURI="folder:PermissionSet"/>
</t:AdditionalProperties>
</FolderShape>
<m:FolderIds>
<t:DistinguishedFolderId Id="calendar">
<t:Mailbox>
<t:EmailAddress>someuser#company.onmicrosoft.com</t:EmailAddress>
</t:Mailbox>
</t:DistinguishedFolderId>
</m:FolderIds>
</GetFolder>
</soap:Body>
</soap:Envelope>
Everything is working fine, as I get a response looking like this :
<?xml version="1.0" encoding="utf-8"?>
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
<s:Header>
<h:ServerVersionInfo xmlns:h="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" MajorVersion="15" MinorVersion="20" MajorBuildNumber="56" MinorBuildNumber="13" Version="V2017_07_11"/>
</s:Header>
<s:Body>
<m:GetFolderResponse xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types">
<m:ResponseMessages>
<m:GetFolderResponseMessage ResponseClass="Success">
<m:ResponseCode>NoError</m:ResponseCode>
<m:Folders>
<t:CalendarFolder>
<t:FolderId Id="some-folder-id"/>
<t:DisplayName>Calendar</t:DisplayName>
<t:TotalCount>67</t:TotalCount>
<t:ChildFolderCount>2</t:ChildFolderCount>
<t:PermissionSet>
...
</t:PermissionSet>
</t:CalendarFolder>
</m:Folders>
</m:GetFolderResponseMessage>
</m:ResponseMessages>
</m:GetFolderResponse>
</s:Body>
</s:Envelope>
Except when someuser#company.onmicrosoft.com shares his calendars with an external contact, then when I execute the same request I suddenly get this response error :
<?xml version="1.0" encoding="utf-8"?>
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
<s:Header>
<h:ServerVersionInfo xmlns:h="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" MajorVersion="15" MinorVersion="20" MajorBuildNumber="56" MinorBuildNumber="13" Version="V2017_07_11"/>
</s:Header>
<s:Body>
<m:GetFolderResponse xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types">
<m:ResponseMessages>
<m:GetFolderResponseMessage ResponseClass="Error">
<m:MessageText>Access is denied. Check credentials and try again., Can't look up the requested Entry ID.</m:MessageText>
<m:ResponseCode>ErrorAccessDenied</m:ResponseCode>
<m:DescriptiveLinkKey>0</m:DescriptiveLinkKey>
<m:Folders/>
</m:GetFolderResponseMessage>
</m:ResponseMessages>
</m:GetFolderResponse>
</s:Body>
</s:Envelope>
I don't understand why does Office 365 return an EWS error instead of filtering the data which we should not have access to... We should be able to get the calendar permissions concerning the internal organization regardless of any external calendar sharing.
Anyone having the same issue or info about scheduled resolution ?