Shoulder milling with helical cutters

I am new to CNC machining, but.....

Trying just to make long shoulder 90 degrees, long like 6" deep in Z. Nothing difficult, right.
To avoid million times stepping down I bought several helical cutters, like Sandvik R390 family, one of them R390-032C5-54M. Adding them to Capto extensions and head make really rigid connection, on long distance.

I have Fadal 4020. Speed 2850 RPM. Cutting Feed 880mm/min
The Radial dept of Cut is First 0.4mm, Second 0.4mm, third Finish at 550mm/min 0.2mm.
Material is Al6061T6. Inserts on the cutter are as they should be 100%.

All above looks fine. NOW THE PROBLEM.

I first made a path with only 9mm Step Down. To be modest, not to push it. The first photo is what I got?????? I sent that to Sandvik, the answer, use full length of the cutter. In the case of R390-032C5-54M that would be 54mm deep.
OkieDokie, I have plate of 50mm thickness, should be perfect. Look on the other photos. WHAT IS WRONG?

It's hard to tell from your photos, but it really looks like you have some different inserts in the bottom rows of your cutter, as in not R390 but some APKT's or something.

Edit: a picture

I agree with the above ,my guess is that you're not using the correct inserts.

I think a lot of cutters like that take a different insert on the bottom row that has corner radius. Like others said my bet is it has something to do with inserts. I always thought cutters like that were for 50+hp spindles

What does the worst offset measure? If its .02" or so it could be from using a straight edged insert instead of one with a crowned cutting edge. Its a bit difficult to explain, but because the cutting edge is "tipped back" instead of straight up and down, it has to be in the form of a helix instead of a straight line. Some tool companies recently came out with that for aluminum endmills so they leave a perfect wall with minimal steps.

The other thing is, you probably won't get a perfect wall due to insert variation and/or runout.