When debugging the difference between 15.5 and 15.4 it appears that in the javascript of InplaceEditing, 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. It also explains why the second call works: now the form contains all info. In 15.4 the call is properly performed before the AJAX call. What puzzles me is that I don't see change in xwiki-platform that would explain that between 15.4 and 15.5.
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