The .htaccess files generated during cron through \Drupal\advagg\Asset::generateHtaccess are missing the "E=no-brotli:1" flag, resulting in serving double Brotli-compressed assets.
The content-type header for gzipped assets has also been statically set to "application/javascript" when it should interpolate the function-scoped $type variable.
Originally, I was experiencing what I believed were corrupt assets as noted in the Compression Option's "Gzip" settings toggles, which reads "This should be enabled unless you are experiencing corrupted compressed asset files." This was due to the double-compression of assets, and resulted in the compressed files being served in plain text.
This may be a fix for that issue, as well.
Enable Advanced Aggregation and run its related cron job. Check the resulting .htaccess files located at public://js/optimized/.htaccess and public://css/optimized/.htaccess.
Replace RewriteRule \.{$extension}\.br$ - [T={$type},E=no-gzip:1] with RewriteRule \.{$extension}\.br$ - [T={$type},E=no-gzip:1,E=no-brotli:1].
Replace RewriteRule \.{$extension}\.gz$ - [T=application/javascript,E=no-gzip:1] with RewriteRule \.{$extension}\.gz$ - [T={$type},E=no-gzip:1,E=no-brotli:1].
Replace relevant .htaccess directives.
Needs review
6.0
Code
Where do we stand on this? This is a fairly critical change in my eyes. Six months of inactivity and I would love to see some movement here.