Groups disappeared in Outlook?

This is an old thread but I just struggled with this and ended up contacting Microsoft Support after nothing I tried worked. It turns out that there is an attribute that controls whether or not the group is hidden from Exchange Clients. This sequence of PowerShell commands fixed it for me for this specific group. You will have to set each group's HiddenFromExchangeClientsEnabled attribute to false to see them all.
PS C:\> Install-Module -Name ExchangeOnlineManagement
PS C:\> Import-Module ExchangeOnlineManagement
PS C:\> Connect-ExchangeOnline -Credential $UserCredential
PS C:\> Get-UnifiedGroup -Identity "Support" | fl HiddenFromExchangeClientsEnabled
HiddenFromExchangeClientsEnabled : True
PS C:\> Set-UnifiedGroup -Identity "Support" -HiddenFromExchangeClientsEnabled:$false
PS C:\> Get-UnifiedGroup -Identity "Support" | fl HiddenFromExchangeClientsEnabled
HiddenFromExchangeClientsEnabled : False