Thank you for reaching us!
I am sorry to hear that you have issue with the captcha settings in Azure AD B2C.
I have tested your scenario in my lab, and I was able to enable and disable the captcha option for both the sign up and sign in flows and the applied function is working accordingly without any issue.
Could you please try to clear the browser data then try to unselect the captcha option again. If that doesn't work, you can try using a different browser. if there any extensions like ad blockers please disable.
If the issue has persisted, please try to create a new user flow with a new application and choose the captcha option again and see if it works.
Hope this helps. Do let us know if you any further queries by responding in the comments section.
Thanks,
Akhilesh.
If this answers your query, do click Accept Answer
and Yes
for was this answer helpful. And, if you have any further query do let us know.