Can confirm that this only happens to me when I'm in the unending coil of bahamut or 24 man raids...
My whole raid group also experienced it,
for example tonight a twister stopped casting at about 75% and then popped below us all - as in the cast bar just disappeared at 75% and boom.

As a paying customer I find it appalling that the issue hasn't at least been acknowledged in a forum post. This has been lasting since the 4.15 update, a long time to have such an issue ignored.

I'd like to at least get explanations during the live letter ??

To me it seems like weak server capacity..