I have only noticed this in hero testing. Most noticeable on haze since her spray pattern is almost purely vertical, but it happens to wraith and infernus if not other characters too. Basically I've noticed the bullet spread become random if I have a really high fire rate. I noticed it most when I proc mercurial magnum because the bullet tracers gain that extra purple element which starts going off in different directions so it's really easy to see, but it seems to happen with any really high fire rate. In the example photo I uploaded, you can see the vertical spray pattern with a lower fire rate, and the much more inaccurate spray pattern with the higher fire rate after proccing mercurial. I think this example was on Infernus, but I had similar results with haze and wraith. I haven't actually noticed this in a real game but it might still be happening and I just haven't paid attention to my bullet tracers while fighting. It feels like bullet spread just increases naturally with fire rate, I don't think it is isolated to mercurial.
Another similar issue, the hit audio is a bit buggy as well when the fire rate gets too high. This might even be because of the weird spray when the fire rate is really high though so who knows.

Here is another example I did using wraith with things like vamp burst and full auto and fury trance. The bullet holes started to fade, but the left-most set of bullet holes was from the max fire rate that I managed which was ~330%, and the right-most set of bullet holes is 10% fire rate bonus because reset hero only reduced the cultist buff to 10% instead of removing it altogether.

Not sure if this is intentional, but it kind of sucks since it's just fully random spread and not a spray pattern you can adjust for.
Another similar issue, the hit audio is a bit buggy as well when the fire rate gets too high. This might even be because of the weird spray when the fire rate is really high though so who knows.

Here is another example I did using wraith with things like vamp burst and full auto and fury trance. The bullet holes started to fade, but the left-most set of bullet holes was from the max fire rate that I managed which was ~330%, and the right-most set of bullet holes is 10% fire rate bonus because reset hero only reduced the cultist buff to 10% instead of removing it altogether.

Not sure if this is intentional, but it kind of sucks since it's just fully random spread and not a spray pattern you can adjust for.