When merging, [44244] and [44260] were committed to trunk the opposite way that they were committed to the 5.0 branch. They were originally committed in [43885] and [43861], respectively.
Due to this switch, a change in how the Custom Fields meta box was registered wasn't merged from the 5.0 branch, causing it to not be registered correctly in trunk.
Props dd32, pento.
Fixes#46028.
Built from https://develop.svn.wordpress.org/trunk@44648
git-svn-id: http://core.svn.wordpress.org/trunk@44479 1a063a9b-81f0-0310-95a4-ce76da25c4cd