Hi @Nanjappa, Ashwini,
Per my test, I could reproduce your issue by using the json formatting twice. I could hide the button when display the code first time.
But after delete the code and apply it again, the code doesn't work any more. And the New button background color changed while there is no code refer to the New button.
Since this issue need to be checked from background, and our forum doesn't have access to the backend currently. The best way to troubleshoot the issue is to report the issue to M365 admin center. The support team there has the correct escalation channel for this case. This is also the most efficient way in handling this kind of issue.
And as a workaround for this issue, you could reference third-party CSS styles in SharePoint Framework Extension/ web parts
Here is a ready-made solution in github
https://github.com/pnp/sp-dev-fx-extensions/tree/main/samples/js-command-hide-commands
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.