OEM Aeron Classic pistons have a taper to them that fits snuggly onto the base, the cheaper ones don't have the taper so it doesn't sit low.
I bought the piston from amazon with shipping + tax
Pictures? Have you contacted Herman Miller? Even if it’s outside of the 12 year warranty (or whatever it is) for free replacement of parts, I’m guessing they may still be helpful enough to share likely causes or something. I’d probably just buy a new cylinder, personally. $60 on Amazon.
Based on only your description here, I’d venture a pure guess that the internal locking mechanism (inside the cylinder) has failed. But if that were true, I’d assume it would only return to maximum height, and not any given midway point at which the lever is released.
Looks like just the rod in the cylinder is broken. Replacing the cylinder is all you need to do. I recently did it on my classic Herman Miller Aeron classic(pre 2005) size C with this
Amazon.com: New Pneumatic Cylinder for Herman Miller Aeron Chair #1B08VM: Home & Kitchen
Works perfectly. Just know that getting the cylinder out of the base is a pain the ass. Use a regular hammer(a fairly heavy one) and bang it out. Rubber mallets may not be sufficient as that is what I tried with at first. To get the rod out of the top portion you'll want to use a pipe wrench to twist it out.
I used this Pneumatic Cylinder, and It's held up amazingly well over the last year it's been in use.
Also, do yourself a favor and get a 24-inch pipe wrench to turn that sucker free. I'm a larger guy, and the 12-inch wrench I had at home wasn't big enough to get a proper grip. You can pick one up for ~$15 at a hardware store or ~$30 on Amazon.
This is the video I used to replace mine: https://www.youtube.com/watch?v=ivuOxLxuQzo
This is the one I've found that is a direct replacement. The other cheaper ones will sit higher, etc. https://www.amazon.com/gp/product/B07QGCW421
You'll also want a small sledge and a 13-18mm socket depending on the year to beat that old cylinder out.