One thing that I noticed is for RichTextLabels is that you need to have a custom fixed size for all the text to show. For labels, this isn't the case and it gets resized automatically based on the content inside. Is there a way for RichTextLabels to be resized automatically by their content?
One trick I did was check if the scrollbar was shown and increase the size of the RichTextLabel by small increments until the scrollbar is hidden. This causes a little bit of weird user interface displaying though and if done in a while loop, can be blocking :( (if the RTL is large :P)