When detail views are created with Solid Edge API, default labels are assigned: "A" for 1st detail envelope, "B" for 2nd detail envelope and so forth.
Unfortunately, it's impossible to set custom label to detail envelope. The setter of the following property does not work:
At the same time, setters of the next properties work fine:
I can only confirm what you are seeing. Unless anyone else has something to offer, I would suggest submitting an Inicdent Report (IR) to GTAC.