I recalled this incident which pushed me to finally rollover the Empower 401(k) to Schwab IRA. For years I noticed that I had post-tax contribution in the report. This was provided by megacorp as part of a settlement over fees. I wanted to withdraw just that, and any associated growth. Even though it was clearly tracked, a withdrawal would come partially from that, and elsewhere, according to customer service.
So I got the very-long rollover form, filled it out, got to a notary, and uploaded. What led me to do that was the option on the form to receive two checks, and yes, one was for the post-tax and associated growth.
I received the FBO check and drove to the local Schwab office. By the time I arrived home the check had been deposited to my account.