Master edition not being downloaded or installed

I have recently purchased for my two sons the Master edition of Minecraft. The have been happily been playing the standard edition for some time

When I go to Microsoft store I see that I own the update so I am asked to download it. Immediately on hitting the key the download begins and lasts for one second before disconnecting.

I have been in contact with both Microsoft and Xbox, and neither can help.

They suggested that you might be able to

 

Can you help?