Hello GauravTatkar,
Happy to help!
I did a quick test to verify the behavior you encountered. I had no issues with changing the Paragraph Format of the Text on the image block, and I could switch between formats while retaining the font without any problems. Here's a recording of my test. I also observed the same result when testing on a Flashcard block:
To troubleshoot the behavior, would you mind testing using any of the built-in fonts to see if it is font-specific? If the issue isn't font-specific, please open a case with our support team here so we can check what's happening.