When added a rich text editor in a modal dialog , i dont see Rich text editor option itself when Modal Dialog is launched. Using UI Inspector , i see the control is there, but its not visible. 

Steps : 

1. Create a flow action. 

2. Create a section. 

3. Add Rich Text Editor in the section. 

4. Add a button and configure it show local action as modal dialogue

5. Refer same flow action in the modal dialogue. 

6. Launch the flow action

When flow action is launched, we can see the Rich Text Editor Present. 

But when the button in Clicked, We dont see the Rich Text Editor. Others fields are visible though. 

Please check the screenshot attached