Hmm, this is what I have not tried to do, I have tried various options with variable names in the files themselves. I assumed that Terraform Cloud itself adds the necessary prefixes to the variable names that are specified in its backend, in addition, there are no warnings in the backend about this 
But I will try and write about the result.
At the same time, it seems to me that this looks like a bug, either it is a bug in some procedure, or it is a bug with sensitive variables, perhaps they are not exported, I will check this too.
I use Terraform v0.14.3, this is the latest version at the moment:
