Works fine here.
I opened the project you sent me and, as you said, there is no responsive layout. In Outputs in the project sent, I selected the responsive layout and in the dialog I selected the Azure Blue layout and generated an output that looks complete. The only thing that is different is that I had to do it in 2017 as I don't currently have 2015 installed. That machine is on its way back to Microsoft.
Then I upgraded the project and its still OK.
Try those steps in your 2015 project. Open it, select the Responsive Layout, select Azure Blue and generate. What happens there?
If that looks OK, open 2019, click Upgrade RoboHelp project and browse to the XPJ file in 2015.
Let me know what happens.
See www.grainge.org for free RoboHelp and Authoring information.

@petergrainge
Just got off the phone with RH tech support. It ended up being a corrupt TOC. We created a new one and it was fine. Some missing files (we were working with a copy).