CustomSelectControl: downshift warning in console about uncontrolled "selectedItem" prop

[andrewserong]

Description

Following on from #34520 this issue is to track a warning from downshift about selectedItem switching between being a controlled vs uncontrolled prop. It occurs in the global styles for the Site Title block when switching between different types of Appearance.

I previously made an attempt to fix it as part of #34520, but to get a shorter-term fix for in, I've created this issue to track the follow-up for this warning. See discussion at: #34520 (comment)

Step-by-step reproduction instructions

With #34520 applied (so that you're able to select

1. Activate the TT1-Blocks theme (which sets a font weight but not a font style attribute for the Site Title block)
2. Open up the site editor and the global styles panel
3. Open up your browser console
4. From the global styles panel, select the Site Title block, and adjust the appearance between the default value and any of the real values. After switching back and forward a couple of times, you should see the error message in the screenshot

Screenshots, screen recording, code snippet

Environment info

• TT1-blocks theme

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes