I couldn’t solve the problem. I had a zipped up folder of the game for what I had when I recently upgraded to the UE 5.6 version which had the town build out a bit before I finished the town and started testing it.
I went back to that and put nav meshes down first to test and didn’t have that problem. I opened up the previous project and selected the meshes from the top perspective and copied and paste them. I had to migrate the structures output from the build tool to copy some of the walls and floors which were not there.
I don’t have the ‘can’t get past a certain Y axis point’ problems but it looks like the meshes I have made with the build tool don’t have collision so I will have to fix that.
Looks the initial problem I had will be an unsolved mystery. It is a good practice to zip up the project ever so often or have it on source control to go back to a version which doesn’t have the problem and attempt to recreate the level without the problem.