I was trying to create a couple of parallel vias connected on both the top and bottom layers but whenever I linked the 2 vias on one layer, the link on the other layer would disappear. So, I tried locking one of them.
If I generate the gerbers the isolated bit of track is present. Tools > Cleanup Tracks & Vias doesn’t get rid of it.
I was afraid of that. Must have been a long time since I used to lock mounting holes on the PCB to stop them disappearing when updating the PCB from the schematic. The “Not in schematic” check box has probably been there forever too!