Layers not opening correctly

[Simon0D4D]

I have a random problem occuring during my focus stacking workflow and would appreciate some guidance. Following the tutorial flow of editing in LR then choosing Edit/Open as Layers In Photoshop gives hit and miss results: sometimes PS correctly opens them as layers in one file so I can then proceed with Auto-aligning etc, but a lot of the time some of the layers are in one tabbed document and one (labelled as 'Background') is in a second document - see screenshot.

I am typically using 3-7 images and it seems competely random as to if it works or not. Sometimes it seems to be better to have PS open before I try to open in layers from LR, but this is not consistent.

Both programmes are up to date.

Appreciate any help I can get!

Simon