There are already multiple threads on this topic. Engineering unlock is bugged. It is a kind of meta RNG grind to get your covert missions even counting towards the goal, and we still don't know if it should be 6 in total (CM wrote it that way) or 6 thefts + 6 heists (support responded that way to a user).
I'd suggest to stop grinding towards this engineer unlock and wait for the bug to be fixed. Alternatively, you can give in to the meta grind and try as many covert heists as you can get. I'm sitting at 7 thefts and 8 heists (all covert, all successful, ofc) and never got the invite. Perhaps it unlocks after 20 heists or so for me, who knows.
The good thing is: after around the 10 mission mark, covert gets super easy because you know what you are doing. The bad thing is: after around the 10 mission mark, covert gets super easy and thus boring as hell. At least it is a nice opportunity to murder a whole settlement and loot it.