I've had this issue on a few ponies I've rehaired, I find it's best to boil the head (I always do this but especially with enlarged plugs or small splits) to shrink all of the holes, and then go over the split section with some kind of vinyl glue to fill it in and reinforce the row. I use Liquid Fusion, usually by dabbing some over the split plugs on the outside and the inside, and then I reapply after that layer dries until I'm left with a smooth surface kind of like filling in a crack/hole on a wall lol.