[Astrophotography: pasting synthetic flat back into layers changes the merged layer to a flat as well]

Thanks. I'll try to generate the video as requested. In the meantime, the video I was watching is below...and the action I refer to is at 32:26 minutes into the video. Like him, I used cmd C, closed the file (cmd W) then cmd V, thus I did not choose the position in the stack as such. 

...and the action I refer to is at 32:26 minutes into the video. Like him, I used cmd C, closed the file then cmd V, thus I did not choose the position in the stack as such.

[Astrophotography: pasting synthetic flat back into layers changes the merged layer to a flat as well]

Forgive me if this has come up before but I haven't found it in a search yet.

I am following, to the letter, James Ritson's YouTube tutorial in which he improves an image of the Orion Nebula. All goes well until I copy the synthetic flat back into the layers stack. Upon doing this, the merged (visible) layer (from the raw image plus curves etc) immediately changes to a copy of the synthetic flat. So, I have two synthetic flats and the original merged image has vanished. 

I have tried this procedure twenty times now with the same result. I have even tried the turn off/on idea and even rebooted the computer but, alas, the same happens each time.

I suspect there's a button I may have turned on accidentally or maybe it's because my ancient Mac, running 10.11.6 El Capitan, does work properly with Affinity, although nothing else seems wrong.

Help!