If coming out on the upper branches, it is not a rootstock issue nor is it a permanent reversion. Linearlobum type JM's often will respond with this unusual growth if excessively cold damaged or the leaves have been damaged for some other reason during the course of the growing season. Typically, it is the most robust shoots that will demonstrate this abnormal growth. Next season, provided not affected by winter or bugs, the leaves should emerge with their normal narrow, strap-like form. There is no need to prune off or remove this growth.

Check the shoots emerging from lower on the trunk. If they are coming from below the graft, remove. If not from below the graft, you can remove or leave as you wish.

Thank you so much garden gal! I'm in the same zone as you. In our case the damage was due to a total infestation of those worm larvae. I didn't even see them at first due to the linearfolium shape and colour. The worms were perfectly camouflaged. I hope they don't return next year but they were eating the cherries, plums, Garry oaks, apples and the other mature planted JM's. Very disheartening! I hand removed and sprayed with a Safers product but wow: what damage!!