mailNickname Attribute being Cleared in Hybrid Exchange Configuration
It may be necessary to modify only the mailNickname attribute on an object when in a Hybrid Exchange configuration with Office 365. When doing this in this configuration, the mailNickname attribute gets cleared out.
This is due to the builtin policy Built-in Policy - Default E-mail Alias. This is meant for a regular Exchange configuration where it will build the mailNickname attribute based on other Exchange properties. In the case of being in a Hybrid Exchange configuration, those source properties may not be filled in so the mailNickname attribute is cleared out.
Disable the builtin policy Built-in Policy - Default E-mail Alias. You can create your own policy to generate the value of the mailNickname attribute, if need be.