And apparently I'm seeing the regression between 16.4.2 and 16.4.3 so it would be something done in both 15.10.12 and 16.4.3.
https://github.com/xwiki/xwiki-platform/commit/f7630ae8d671bb6d9e96ad2fdb8d46b4b1ac4548 might be a good lead for the regression.