I’ve attempted to recreate your issue… I built a new WP site with the 2019 theme and no plugins… I set up my block/post editor similar to your screenshot, but I cannot reproduce the fading issue.
A couple more follow-up questions…
(a) Are you on the latest release of WP? (Dashboard, bottom right… 5.7.2?)
(b) What browser (and browser version) are you using? I’m in Chrome Version 91.0.4472.114 (Official Build) (x86_64) using an Apple MacBook Pro
I wonder if it is possible that you have a browser extension (e.g. if you are using Chrome or Safari), that is modifying the text. For example, something like the Grammarly extension, etc. It *shouldn’t* affect your experience, though, unless there’s an extension that purposefully highlights/focuses on text blocks.