Hi @Pete Bye ,
Welcome to our forum!
According to you description, i understand the issue only occur in AVD, i'm afraid that the issue ie related to some settings in AVD. Based on my research, i found that when the below Group policy is enabled, users cannot change cached time period. Please contact the administrator to check if the Cached Exchange Mode Sync Setting is enabled and set to One month.
If so, please disable it.
Hope to help you!
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