Some mailboxes list as user mailbox in Exchange 2013 while list as Shared mailbox in Exchange Online

Question

Hybrid setup. EX 2013 CU 23 with all updates. One single server, no DAG etc. All is migrated to O365.
A few random mailboxes list as User mailbox in EX while correctly list as Shared mailbox in EXO. Or the other way around!
We have Azure AD Connect 2.1.1.0 which was upgraded from previous versions except 1.0.88.0 which was never installed.
I have manually updated the AD object's msExchRecipientDisplayType, msExchRecipientTypeDetails, msExchRemoteRecipientType and userAccountControl, and thus resolved this however would really like to know if anyone has any ideas why this would happen in the first place.

example of code used to set mailbox to be of type remote shared mailbox

Set-ADUser -Identity ((Get-Recipient <mailbox>).samaccountname) -Replace @{msExchRemoteRecipientType=100;msExchRecipientTypeDetails=34359738368;userAccountControl=66050}

example of code used to set mailbox to be of type remote user mailbox

Set-ADUser -Identity ((Get-Recipient <mailbox>).samaccountname) -Replace @{msExchModerationFlags=6;msExchProvisioningFlags=0;msExchRecipientDisplayType=-2147483642;msExchRecipientTypeDetails=2147483648;msExchRemoteRecipientType=1;msExchTransportRecipientSettingsFlags=0;msExchUserAccountControl=0;userAccountControl=66048}

Although this resolves the issue I do not understand why an mailbox suddenly would list in EX on-premises as User while it list correctly as Shared in Exchange Online (or vice versa).

Answer 1

Thats actually very common.
It all depends on how it was created and sometimes things go sideways. If the mailbox was converted to a shared mailbox in Exchange Online for example.

I have also see remote mailboxes set to a user mailbox on-prem but not accurately updated from a shared to regular in Exchange Online.

I have scripts that check for that as a work-around.