My thoughts are that the engine in a motorcycle is usually fixed to the frame forming part of the structure as it were, in the Morgan it is on engine mounts.
The front engine mounts are in a cantilever position on the frame with constant pulses and vibration from the engine (the front mounts are further away from the joint than the rear also), with the lower weld joint on the upright taking the punishment, which I believe may be causing the cracks here no matter what year the chassis, it really needs some triangulation or extra support at this joint.
Just my thoughts but it doesn't seem right that half the engine etc. weight is taken up by unsupported tubing with no webbing.