Yes I started to investigate it, but never found time for properly fixing it. Removing myself from assignee for now. @CamilleDesmots do you have double underscore in the username? I believe that if you experienced it only now it might be another problem as XWIKI-19670 was not a regression.
I’m not sure if it’s related, but we were seeing a similar problem with the “like” button on pages; swapping to a different icon theme (Font Awesome) from the default appears to have fixed that problem.
This is a new installation using Docker on v14.10.3