Hello,
after some hours of investigating in UE 4.26.2 and with Substance 2021.1.1 and frequent crashes:
there is a bug when exporting to Substance.
Especially when renaming the “slot name”.
I did not find any logical system right now, sometimes it has the name of the original material sometimes not.
Sometimes it is called lambert, but there is no lambert when opening in May.
Or Worldgridmaterial in Substance.
I would not care about if there would not be these crashes at export from UE4 to SP, which are related to the naming and material.