@Chris, no guarantee that I have a solution.

I had to remove all pop-ups and then I applied pop-up image to the WB and then removed it again.
Eventually I got all borders removed.

When I added pop-up image to individual pictures, no border appeared.
I can but guess you hammered the design so much it got confused.

On the Mobile view, I dragged all the pictures off-page (ghosted them too), deleted the empty FlexGroup and make the remaining BlockBackground 1px high.
This remains the Main WB pictures and all WBs could be repositioned as normal.

Acorn