Just experienced the same issue after putting a Text Component in a Retainer Box. Updating the text wasn’t updating the appearance.
I tried everything in this thread, and 1) Calling Request Render on the Retainer Box did nothing. 2) Setting Phase Count to 2 makes it update, but inconsistently (sometimes it updates when the text changes, sometimes it doesn’t)
Finally, I found a solution. Call the ForceLayoutPrepass function on the Text Component (not the Retainer Box) after updating the text contents and this should cause it to update.
Render Rules are set to Retain Render (true), Render on Invalidation (false), Render on Phase (true), Phase (0), and Phase Count (1).
By the way, another issue I noticed is that the visual appearance can vary significantly when Retain Render is on compared to when it’s off. The solution in the following thread of adding gamma adjustment to the Retainer Box’s material worked for me, although it looks like in the latest UE version (5.1) you might get better results by taking the power to a value between 0 and 1, and maybe also including some multiplication to adjust the appearance as appropriate.