Per the attached diff, the following line was removed from attachment-edit.vm between versions 4.2.1 and 4.2.3 (at least for the EAR/WAR version):
#customControlFooter ($action "attachment_field" $fieldLayoutItem.getFieldDescription() $displayParameters $auiparams)
This seems to prevent a closing DIV tag from appearing in any transition screen with the Attachments field enabled which breaks the formatting of the page. This accompanied by the JFS plugin will prevent any fields below the Attachments field from being displayed whatsoever. If I either re-add the aforementioned customControlFooter line or remove the unterminated customControlHeader line, formatting is restored.