Try it on the latest (v9.10.0):
or in the CodeSandbox:
I can take a shot at fixing it!
thank you @wajeehsheikh
So after looking into this bug. The solution is, for the disabled button to add the disabled prop in the <Input /> tag like this:

Instead it adds the disabled prop in the </FormControl> tag like this:

According to the disabled props description, "Displays label in light grey if true"
Which it does. So is this the intended purpose?

@wajeehsheikh I believe @NameFILIP 's point was that the FormControl component should automatically propagate the disabled property to its' children - is that correct?
Correct. Otherwise, I need to pass the disabled property to both FormControl and Input which is redundant