From slack:

there appears to still be an issue dokku network:set --global initial-network app-internal ! App name must begin with lowercase alphanumeric character, and cannot include uppercase characters, colons, or underscores it thinks that app-internal is an app name, where it's actually a network name

The problem caused by network.CommandSet which validates the appName with common.VerifyAppName. Before calling this method from the subcommands the appName is replaced with --global which isn't a valid app name. I applied the logic I saw in common.CommandPropertySet to skip the verification when the appName parameter is --global.