If i set to required: true, the "*" does not appear in the label.
If the richtextfield is required: true, and if other field in the form failed the validation, this richtextfield, even if it has value, will show error. This only happens when you are creating the data. Updating the data will not encounter this issue.
As reported by a user on our forum https://forum.magnolia-cms.com/forum/thread.html?threadId=8c6c0b0f-380d-4e91-be56-c7528856b5f6 and verified personally by yours truly.