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This is a predictable phenomenon. According to this blog: https://techcommunity.microsoft.com/t5/exchange-team-blog/change-in-naming-convention-of-user-s-name-parameter/bc-p/3672755
We’ll use this GUID as Name instead of synchronizing the Name from on-premises (when in Hybrid environment) or using the alias (if Name is not specified) to compute the Name parameter in Exchange Online.
You’ll need a roundtrip to PowerShell and use the Get-Recipient cmdLets to obtain the DisplayName of the GUID shown in your mailbox permissions, such as:
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