After update from WP 5.0.2 to 5.0.3 I had issues in wp-admin. I couldn't edit post data. There was JS error "Uncaught SyntaxError: missing ) after argument list", undefined "_" (lodash) etc.
The issue for me was the
clean_script_tag() function from Soil.
add_filter('script_loader_tag', 'clean_script_tag'); solved the bug for me.
I found that:
Note: wp-polyfill-fetch.min.js?ver=3.0.0 ' defer " >
Reason: Confusion/conflict with quotes (' & ") witch break down JS.
The solve: Delete/disable the adding 'defer' PHP-hook or add some conditions for don't touch Gutenberg scripts.
More detals here: https://github.com/WordPress/gutenberg/issues/13029