In the first image you can see that when I RMB on the first assembly there is an icon showing that the assembly is set to Flexible. If I click it the assembly will be set to Rigid. This is the way it is supposed to work.
Now when I RMB on either of the other two assemblies that icon in the popup is missing. What gives? It is not a case of that option being unavailable either, because if I go to the Properties icon in the far right corner one of the many things available is the option to solve the assembly as Rigid or Flexible.
I am sorry, but I cannot share these files.
I am using SWX2018 SP2.0. I will be checking more assemblies today to see if I can figure out what is going on, but wanted to put this out for others to see and perhaps verify if you are/are not seeing the same thing.