Editing font-size in Paragraph or Heading block causes error on page

Providing an update here.

If I use the slider in the side panel to adjust the size, it gives this error:
Uncaught TypeError: fontSizeValue?.startsWith is not a function

If I use the text input in the side panel to adjust the size, it gives this error:
fontSizeValue?.startsWith is not a function and also says "This block has encountered an error and cannot be previewed."

I can hide those with CSS or JavaScript so the user doesn't see it.

I also tried making my own heading and paragraph block. Everything works except the default behavior of when user presses enter key.

I'm also experiencing it. It seems to be related to a bug in the Gutenberg library. https://github.com/WordPress/gutenberg/issues/62072

As a workaround you can define predefined font-size as described in https://www.drupal.org/docs/contributed-modules/gutenberg/creating-custo... →

That will "fix" the problem.

I reviewed this issue and the MR applied successfully for me, and the changes fixed the issue for me as seen in the attached video.
The changes looks good to me thus moving it to RTBC+!