Steps to reproduce

1. Visit https://bitbucket.org/atlassianlabs/snap-json-logging/admin (I've given admin permission to atlassianlabs:ad-managed-write-repositories).
2. Untick "This is a private repository¹"
3. Click "Save repository details"

Observed

The form submits and says changes saved, but the setting remains ticked and the repository remains private.

Expected

The setting is unticked and the repository is public.

Notes

This has the same symptoms as BCLOUD-14712. It's been replicated by another user using Chrome; I'm using Firefox.