Enabling Yammer Preview is affecting access to Yammer due to a conditional access policy we have configured.
The conditional access policy requires MFA authentication for browser access to Exchange Online (Outlook Web) from non-trusted locations. Two different version of the conditional access policy have been tested; * using only Exchange Online as target cloud app (with and without excluding Office 365 Yammer from the list of cloud apps) * the new Office 365 (preview) app and excluding Office 365 Yammer. Both policies result in MFA requirement when Yammer Preview is enabled. Yammer Classic is not affected by the policy. Different browsers tested. There are no service dependencies listed for Yammer Preview in https://docs.microsoft.com/en-us/azure/active-directory/conditional-access/service-dependencies.
A support ticket has been opened with Microsoft (although preview is unsupported), but posting here as well just in case someone runs into the issue.
We had to ask users to revert to Yammer Classic to prevent being prompted for MFA.