@pinacolada Ok so I think I've resolved this.
Firstly, I took advice from a YouTube video which described how to update RetroPie itself - this took me to the following screens:
Update Remote Setup Script
Basic Install
Perform Reboot
It took a while, and looked like some errors occurred during the install, however...it did come back up, and now the hotkey + right trigger DOES load from the save slot.
So I guess the root cause here was an older version of Retropie.