When adding device to QtCreator from "Edit" > "Preferences" > "Devices" > "Add" and add it with default authentication types. It ends up situation where "Authentication type" radio button show "Default" and "Private key file" field is available:

If user later wants to add key to device, it's reasonable to try addition straight to "Private key file" field without another changes. Instead also "Authentication type" is needed to set "Specific key" value.

After changing "Authentication type" to "Specific key" and then back to "Default" seems that "Private key file" field is not available.

"Private key file" field is expected to be always disallowed when "Authentication type" is "Default"