Sometimes when i use "Play another Mission" after i played a stronghold i can't Play another stronghold, cuz it says unlock it by finishing "Finding old Friends" even though it is unlocked and shown as completed in the journal. This might as well be the reason why the Legendary Freelancer achievement (complete misssion challenges) doesn't tick off.
Haven't encountered the problem myself, but this sounds like it's worth a bug report if there isn't already one covering the issue.
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