hi! I was doing /plot set floor dirt on one plot that wasn't merged, and When I did this, it merged into the roads, and another persons plot. Now I can edit their plot and the 3 roads. The plot ID I did this from is -12;24.
It seems like your plots were merged but there was an issue with the road not being removed. I would just suggest un-merging and deleting the plots and re-merging them.