That is possible, but they seem well healed to me. I think it is mostly because there has just been so much continuous rain, plus soil splashed up along the stem as well, pretty much caking them in wet soil conducive to rooting. I have taken the clips off all of them and rubbed the stems to knock off any roots and they are sturdy. I actually have 3 grafted plants still in the starter cells and they are out of the rain and showing no signs of rooting so the rain and possibly the wet dirt on the stems appear to be what is causing it. Also there are roots coming out of the rootstock stems as well so the moisture is basically making the whole garden into a misting chamber.
But I think the scions are also predisposed to root out at this point since I had rooting start on many of them while they were in the healing process, probably because I kept them under plastic for too long (6 days) and was spritzing them with a spray bottle every day for the first three days. I expect the extended humidity combined with the water getting trapped by the clips lead to the rooting starting before they were uncovered. Next year I’ll be more vigilant about getting them out from under the plastic and not baby them so much even if it means a few more don’t make it since I think the rooting is a real problem and once you create the cells differentiated to root cells it is more likely to happen again. Next year I will also graft them higher instead of cutting the rootstocks below the cotyledons so the scion isn’t as close to the ground. The videos I watched suggested cutting below the cotyledons to avoid rootstock suckers but with my few plants I can easily just watch and pinch out any that emerge.
In terms of size of the scions, I basically used this video as a guide for how big to grow the plants and how much foliage was on the scions.
I planted the rootsock and scion plants in ProMix BX and had 15 of 18 survive. After grafting the complete plant was about 4-5 inches and now they’re close to 6 inches, greened up and growing.