It looks like there might be a bug when filtering on certain fields. Internally, Ghost has a posts table, and a posts_meta table. The posts_meta table was added a few years ago because of column width limitations on the posts table.
When filtering a native column, the filter is properly translated (e.g. %ghost%). but when filtering a joined column, the filter is not translated, (e.g. /this/i).
# Filter: filter=html:~%27Ghost%27
# Result: 7 posts
knex:query select id,uuid,title,slug,html,comment_id,plaintext,feature_image,featured,type,status,locale,visibility,email_recipient_filter,created_at,created_by,updated_at,updated_by,published_at,published_by,custom_excerpt,codeinjection_head,codeinjection_foot,custom_template,canonical_url,newsletter_id,show_title_and_feature_image from `posts` where (`posts`.`status` = ? and (lower(`posts`.`html`) like ? ESCAPE ? and `posts`.`type` = ?)) order by `posts`.`published_at` DESC limit ? undefined
knex:bindings [ 'published', '%ghost%', '*', 'post', 15 ] undefined
# Filter: filter=meta_description:~%27this%27
# Result: 0 posts
# Expected: 1 post
knex:query select id,uuid,title,slug,html,comment_id,plaintext,feature_image,featured,type,status,locale,visibility,email_recipient_filter,created_at,created_by,updated_at,updated_by,published_at,published_by,custom_excerpt,codeinjection_head,codeinjection_foot,custom_template,canonical_url,newsletter_id,show_title_and_feature_image from `posts` where (`posts`.`status` = ? and (`posts`.`type` = ? and `posts`.`id` in (select `posts`.`id` from `posts` left join `posts_meta` on `posts_meta`.`post_id` = `posts`.`id` where `posts_meta`.`meta_description` like ?))) order by `posts`.`published_at` DESC limit ? undefined
knex:bindings [ 'published', 'post', /this/i, 15 ] undefined