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?
Big oops. Last patch I fixed a bug with Hail of Blades which caused Jhin to sometimes become unable to 4th shot at all. I fixed this by removing a seemingly superfluous check somewhere that reset that spell slot to his crit auto attack. That change exposed a separate issue that already existed—Jhin can be prevented from reloading sometimes, which places him into a state where he has zero ammo and whatever is placed in his spell slot becomes his crit basic attack until he reloads (defaults to his 4th shot, but can also be his Q or E). Before last patch, this would lead to perma 4th shot. Now, it leads to perma 4th shot or perma Q/E (whichever he cast last). This is now fixed for next patch by allowing Jhin's reload to be castable under all conditions.
And no, you can't get his ultimate to replace his basic attack, sadly.