Thought I'd add a final recap in case anyone searches for a similar problem and finds this thread.
My issue ended up being due to insufficient clamping pressure on the starting edge of the tack. One of the springs that holds down the clqmping feet had been replaced and happened to be on the left (starting) side of the stitch. Swapping the two springs so the stronger one was on the left immediately improved the issue and some fine tuning of tension brought things all perfectly together.
The other factors that maybe were contributing to this issue were a slightly worn and misaligned feed plate. It's a more involved adjustment to re-center it so I had avoided it, but the gap in the feed plate is shifted a little to the left, contributing to less clamping on that left end of the tack. One day I'll get around to it, but for now just upping the clamping pressure was enough.