URichTextBlock::UpdateStyleData() does not check whether DecoratorClasses already had any specificed DecoratorClass. So every time you turn UMG on and off from the Viewport, Decorator Classes will increase indefinitely.
I propose that we add check code to a following code ( ex. URichTextBlock::GetDecoratorByClass )
result : DecoratorClasses of RichTextBlock has 2 instances.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-120641 in the post.