5.8 Issue formatting text in a paragraph block

I have reported this on github too.

When highlighting text in a paragraph block and clicking on ‘B’ to make it bold, only part of the highlighted text changes to bold. If I undo and try again, a different section of the text changes to bold, but not all of it. In HTML view, the ‘strong’ tags have been placed incorrectly within the highlighted text instead of at the start and end of the text.

The same happens if you highlight the text and try to change it to italic, or change the colour. Only part of the text is affected.

I have tested on several websites, with plugins disabled and switching to Twenty Twenty One theme. I reverted to 5.7.2 and the problem disappears – therefore I believe it is a bug in 5.8