"Separate Membership Payment" breaks on recurring membership

I just replicated this on the demo server with the following steps:

• Create a new auto-recurring membership type.
• Added the membership type to a contribution page that allows member signups.
• Added the ability to take additional contributions on the same page.
• Selected the "Separate Membership Payment" checkbox.

This works correctly on non-recurring memberships - but on recurring memberships, it makes the "additional payment" value recurring, not the membership.

For instance, when I created a recurring membership that cost $22/month, and signed up with a $3 additional contribution, it recorded a recurring contribution of $3/month, not $22/month.

I replicated all this on the demo server, but can confirm that on a live server, the wrong data is also sent to the payment processor.