On this diff https://fr.wikipedia.org/w/index.php?title=Discussion_Projet:Zoologie/Quel_est_cet_animal_%3F&diff=next&oldid=144975710
The user in question does not have this problem continuously :
I'm on Windows 10 + Firefox Quantum Version 58.0.1. The problem still persists right now (I just see the Y of "yes" and the final parenthesis) and this for any page (except Flow, of course) whenever I want to thank from a diff. From the history page there is no problem to send thanks.