Sure..I'd be happy to. After I had that problem and the added pixel problem, I stopped working with that file. It still exists, but I tried to reproduce the problem this morning and couldn't. I could try re-creating it from scratch, using the source file again.
I can describe what I did...I created a new Affinity document, and placed a graphic at 3X size, then made 2 duplicates of that on the layer, a 2x and a 1x. The goal was to then export the slices. Somewhere in that whole process, the yellow triangle appeared, and curious that not all of the slices had it!
If I can get it to happen again, I'll zip up all related files and send it your way.