When I was working on unlocking Terra I was told that covert thefts did not count towards progress (confirmed by FDev support as a known bug).
I'm not sure if it was ever resolved.
This was from 9th of August:
Hey CMDR,
Thanks for getting back in touch in regards to Terra Velasquez.
I've managed to obtain some official confirmation from our internal QA team so please let me explain further.
The intended requirements for unlocking Terra Velasquez:
You do either 6 Covert heist missions or 6 covert Theft missions. (So you only need to do 6 of either)
However, currently there is a bug that is preventing the covert theft mission from tracking correctly. Our QA team has been made ware of this and the matter is under investigation.
In regards to your situation, your 6x covert heist missions counted correctly. However, our database did not update in real time to reflect the change and after completing another covert theft mission, the database updated. This was purely coincidental.
Rest assured, a fix is being worked on and we hope to deploy a fix for this soon!
I hope this clarified the unlocking requirements further.
Do send us a new ticket if you have any other questions.
To sum up: To be on the safe side you need to do 6 covert heists. Then relog (and pray*).
*Praying might be optional