Can't make a goup calendar readonly anymore with Powershell

Hi,

We have some groups where regular group members are not allowed to make changes to the group calendar. I was able achieve this with Powershell as described in this post: Calendar permissions in an Office 365 Group - Microsoft Community

More info about the Set-UnifiedGroup command is found here: Set-UnifiedGroup (ExchangePowerShell) | Microsoft Learn

All of a sudden I now am only able to read the settings and can't change them anymore. So this still works:

Get-UnifiedGroup -Identity "MYGROUP" -IncludeAllProperties | Format-List *Calendar*

But this gives an error:

Set-UnifiedGroup "MYGROUP" -CalendarMemberReadOnly

The error is: 

Write-ErrorMessage : Object reference not set to an instance of an object.

The strange thing about this, is that I am able to change other attributes, like:

Set-UnifiedGroup -Identity "MYGROUP" -AccessType Private

I use an account with Global Admin rights to execute the commands and we have an educational tenant. What is going wrong here?