My guess is the culprit is /advanced-custom-fields-pro/pro/blocks.php line 523 if( !empty($query['preview']) ) {. Assuming $query['preview'] is no longer evaluating as empty on the frontend.
Unfortunately, I do not have a local install of wordpress with 5.5 to see what $query['preview'] evaluates to on the frontend as of the latest update and I am currently busy with a deadline.
Good luck with the bug hunting!
Submitting a bug report as I am seeing the same behaviour on 5.5.
I have a live site that relies heavily on is_preview to render content differently in Gutenberg versus the front end. Now is_preview is returning true on the frontend.
