In the latest version you have added the rich text in the portal and it works just fine.
The problem about it is that the "Comments" pop-up is not a form, it is a hardcoded web resource defined on the basic form metadata of type "Timeline". As such, the rich text editor is not available to be added on this pop-up, as we cannot customize it.
Unfortunately the comment entity is not even a “regular” entity, only a support one for this feature. So we wouldn’t even be able to add this relationship. This “comment” option is really a very specific feature and cannot be used as other form fields.
It is very important to us to add this rich text into the comments on the portal because our hundreds of clients and partners need to add several screenshots in the comments (not only one) and also because they want to send a story about their comments, for exemple, when they want to give an specific exemple they describe the case with an image and continue describing with another image so it is clear for the technician to solve their errors.
Please take in account this request for your next version.