I don't think it's the case, but check that the vertical crack isn't wider at the top than the bottom. If so then the far end of the extension would be sinking, i.e. it would be tipping downwards away from the original building.
If it's a consistent thickness from top to bottom then it's probably just shrinkage. Most modern builders don't tooth in the bricks as has been done here, as they know they'll get called back a couple of years later after it cracks. The modern approach is to accept movement will occur and build in a movement joint, i.e. a deliberate neat crack full of flexible material.
You could retrospectively add a movement joint, but it's possible that it's done all the moving it wants to and just needs filling. I had a top-to-bottom crack on an internal wall that's where new and old meet. I got it skimmed over 5 years ago and the crack hasn't reappeared.
If the floor is concrete then it may have sunk, or it may have been badly built by someone who didn't take much care. A few bags of screed could solve it, or you could just decide to not care.