Why does the progress for unlocking The Dweller show 0/5 when I sell to a black market?

I have discovered the engineer The Dweller. In order to unlock him I need to trade on at least 5 black markets. My problem is that whenever I sell illicit cargo to a black market contact, the in-game progress still reads 0/5.

This is what I just performed in Solo:

1. Go to LHS 3872 > Curbeam Hub and buy 2 units of Imperial Slaves from the Commodities Market. (I only really need one, the second is just to give me a second chance at selling)
2. Prepare to jump to Adad. The jump-info panel in the center of the screen warns me that I have illicit cargo on board (which is what I expect).
3. Go to Adad > McAuley City, and dock. When I am on the landing pad or down in the hanger, the warning "Illicit Cargo" appears above my heat signature and fuel gauge.
4. Go to the Black Market Contact and sell 1 Imperial Slave. (This was sold at a loss)
5. Open the in-game Engineering page, see that I'm still at 0/5 progress for The Dweller.

I've seen from many places online that getting The Dweller might be bugged, so some additional troubleshooting:

6. Quit to the Main Menu, go back into Solo, check Engineering page, still 0/5.
7. Quit the game to desktop (but the launcher is still going). From the launcher, Options > Log Out Machine. Click the "Sign in With Steam" button. Launch the game again. Back into Solo. Still 0/5.
8. Close the game and launcher and go outside for an hour. Launch the game again. Back into Solo. Still 0/5.

I've sold illicit cargo to 6 Black Markets (all different stations, but some in the same system), but for most transactions I haven't been doing any quit/reload after selling.

Am I at least doing the right process for buying and selling? Do I need to buy from a Black Market and sell to another Black Market?

Other maybe helpful info:

I've owned the game on Steam (PC) for about 4 months, so I've started playing well after The Engineers was released. I have Horizons (and select that option in the launcher). I do not have Odyssey. Steam and the launcher says my game is up to date, which the launcher shows "v 2021.06.08.269978 0.4.6709.0".

Solution 1:

Note: this answer is based on my own experience. Because this problem was a temperamental bug to begin with, other players may still be effected, or the bug may regress in the future. Elite Dangerous is an MMO, with many players, and the game code changes often. This page may be wildly out of date at any point. If you feel that you are encountering a bug, it's advised to contact Frontier's support team.

I contacted Frontier Support and got a response on Jul 13, 2021. They said:

I'm sorry to hear about you being affected by the black market bug. Our QA team are aware of it and are currently working on a fix, but we don't have any ETA on a fix yet.

So yes, it seems I'm doing it correctly, this is just a very long-standing bug.

However, as Aaron F helpfully commented, this bug was fixed a few days later with a server-side patch. It's important to know that this fix doesn't work retroactively.

This will only affect Engineer progress from this point forwards.

I've been playing for a few days now after the fix has dropped and I'm able to continue progressing in multiple engineers.