Custom URL "login"/authentication not working as expected

[cfpbChad]

I'm attempting to create a login to a custom Salesforce domain using the following command:
dmc login -u https://custom-domain/... org1

However, when I issue that command the login redirect doesn't send me to that custom domain's login, instead it sends me to the standard Salesforce "https://login.salesforce.com/...". This will definitely authenticate me to the custom domain. Am I using the command incorrectly? When I do a "help" on that login command, it specifies that I can us the -u flag/option to point to the URL that I wish to use for authentication purposes.

-- Edit --
There is a "workaround" for this capability. When issuing the dmc login -u <custom URL> orgName command, your default browser will open a tab/window to the standard login.salesforce.com. You can modify the domain that is a part of that callout so that it points to the correct custom domain endpoint. You must leave, in place, the callback components (e.g. token, etc...) that are a part of that URL. However, if you modify the domain for that URL and then continue with the login process, the callback will register with the correct domain.