As soon as I add a second email sender domain to the Workflow Domain setting, every workflow in the account immediately switches to a setting of Sender Domain="All domains" and begins splitting emails across both domains.

My clients want to control which domain workflows use to send emails, but without having to edit the settings of every one of the 100+ workflows in their account as soon as we add a second email sender domain to their account. That's completely impractical.

The Email Services setting on the account has a default domain specified. Workflows should send using the default domain, unless their settings are deliberately changed to select a specific domain or "All domains".