Strange bumps on a plumcot trunk

[Plantinyum]

I have this cross between a plum and an apricot, its a grafted tree from a nursery. Last year it started to form theese bumps on the trunk and along the main scaffold branches. I cut one of them ,there wasnt a worm or other insect in there. They are also not full of resin, ive seen resin pockets just under the bark on most stone fruit species, those are definitely not that
When cut they are firm and look like they're not just a bark formation, but rather have a hardwood core.
It's interesting that the rootstock part doesnt have any of this, they are forming at the graft line and above.
Anyone know what this might be ? Thanks !