For a bit of information on this - Jhin's 'infinite ammo bug' was a thing for a long time already, likely since Jhin's launch. Riot tried to fix it in this patch or last one (and maybe some other stuff too while at it), undocumented, by forcing him to reload when he attacks while under effect of this.
But turns out, as long as he sucessfully rolls a crit, he still gets to shoot and keep the bug active.
And now since the 'fix', all of those shots are 4th shots (which they weren't before)!
And you can also manage to get other spells into the spell slot of his crit attack while under effect of this, such as E!
Ooops?