Question about a chart

I was working a swatch yesterday and couldn’t figure out how to continue with the charted pattern.

In chart A3, which is worked across all stitches in a row/round
Row/round 13 starts with a black square which in the key says this indicates there is no stitch.
At the end of the row there is an additional stitch worked during a YO, double dec, YO.
How do you work this initially?
Move the beginning of the round marker over 1 stitch and then use that unworked stitch (which is now at the end of the round) on the last chart repeat?
Or should the YO, double dec, YO be worked first, using a stitch from the previous round, ie starting the round earlier in order to work it?

And how would it be worked flat?

I don’t think I’ll be making this pattern but not knowing how it is worked correctly is bothering me.

This is really a shift in the marker for the beginning of round and could have been noted in the text. You could knit or slip that stitch and then use it to complete the double decrease at the end of the round.
Worked flat, you could knit the first stitch and then work a yarn over, ssk at the end of row since that will become part of the seam.
The “no stitch” cell in the chart would be better left as an indentation in the row. There’s no decrease in the previous rows as in charts A.1 and A.5 so it’s not actually a no stitch .


Thank you.