the call for filling the content_syntax field is actually performed after the AJAX call for saving in 15.5, which explains the bug: the request is performed with partial info.
The value of content_syntax is set by updateFormDataBeforeSave which is called on xwiki:actions:beforeSave event which is triggered before the AJAX call. Unless your save is custom and for some reason the xwiki:actions:beforeSave event is not triggered?
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