Consider this line documentation of this method:

Here serverCmd.Flags().Lookup("port") returns Flag type.

And Viper.BindFlagValue() accepts FlagValue interface.

The problem is that Flag type does not match FlagValue interface.

One example is Changed method (defined on Flag type) which differs from HasChanged method (defined in FlagValue interface).

Versions:
viper@v1.6.1
pflag@v1.0.5