To reproduce, navigate to Special:Translate with showMessage parameter (from wiki editor or from Special:SearchTranslations): https://translatewiki.net/w/i.php?title=Special:Translate&showMessage=listfiles_date&group=core&language=et. Then next page has optional=1 parameter set, but checkbox for optional messages is empty:
If I navigate further to untranslated messages, then optional messages are shown and checkbox is still empty. Showing an optional message makes sense in case when showMessage value is an optional message, but otherwise it's inconsistent with navigating to Special:Translate without showMessage parameter that results in optional messages being hidden by default.