I've recently been messing around with some thicker aluminum welds and noticed some strange behavior. With 300+ amps available, when I hammer down the puddle seems to get blown back and starts reaching up and out to escape the gas coverage. At higher amperages does the arc try to blow the gas away? It seems like it doesn't like being told what to do above the 250 amp threshold. It tends to act "as expected" if I throttle back and wait for it to let me know when it's ready.
My current theories are:
1. Higher amperage on thick (1"-2" x whatever bar stock) aluminum requires higher gas flow to resist the arc force blowing the shielding (and cleaning) away.
2. Though more horsepower gives the capability to weld thick aluminum, you cannot power through a cold part. My part did not have sufficient time to absorb enough heat. High amperage will act normal on a sufficiently heated part.

I haven't had time to experiment and test these theories but I'm certain there's someone here who can nerd out with me on this.

B.V.

When you start getting into the higher amperage ally welds heat is your friend for sure. Have you tried adding Helium to the argon? This will really bump up the heat naturally without trying to push the machine too much. I will sadly say you do want to move up to a larger size cup over the #5..... With the larger puddle size you'll need the larger gas coverage, too.

Concur with what Crummy is saying here. I have a Dynasty 280 and was welding 1.5" x 1.75" x ~2" aluminum hinges on an aluminum gate and I had to go with a helium argon mix to get enough heat into it. I saw the same issues you are seeing with the puddle getting blown out....especially if I didn't keep a seriously tight arc. I'm a novice though so YMMV.