In JSON, trailing commas (e.g. {"a": "b",}) are not valid. Thus, the templatedata in this old revision is invalid because of the trailing comma after "Description des problèmes relevés lors de la relecture.":
Surprisingly, MediaWiki manages to parse the templatedata in this revision most of the time, but not always. Sometimes, the template data block is not displayed and "Erreur de syntaxe dans JSON." (= "Syntax error in JSON.") is shown instead. I could not find a way to reliable reproduce this.
The parsing behavior should be made consistent.
Note: among the old revisions of the page that contain the error, some have the "PHP7" tag and some others do not, so I have no reason to believe this is related to the HHVM => PHP 7 migration.