This is complicated, and will take a little explaining, but I just had credit removed for the "Javaal Fleet Action Mastery" achievement. What's more, since ALL I lost was the achievement completion, the objectives for the achievement are still completed, and I cannot complete it again, rendering the achievement impossible for me to acquire.
I'll now explain how this happened.
Originally several of the space mission achievements were incapable of being completed, due to extraneous objectives.
Two to three patches ago, a hotfix was introduced which awarded completion of these achievements upon completion of all the objectives which were possible to complete. The downside to this system was that the achievements awarded this way never pulled themselves from your achievement tracker, and you would be stuck with them on your tracker indefinitely, since there is no way to Toggle Tracking on a completed achievement.
This left me with all of my achievements completed, but two achievements left in my quest tracker, Saleucami Fleet Mastery and Javaal Fleet Mastery. And I could not remove them from my tracker (despite sending about a dozen bug reports in-game).
Then, with the last patch, you purged the extraneous objectives and allowed completion of all the Space Mission achievements under normal circumstances. Saleucami Fleet Action and Javaal Fleet Action's trackers were now left with a single objective apiece, one which I had already completed, instead of the previous tracked objectives, which were unobtainable. For Javaal Fleet Action Mastery, the objective was "Neutralize the Enemy Fleet", and for Saleucami Fleet Action Mastery it was "Eliminate Fighters".
So today, I went and did Javaal Fleet Action, and completed the bonus objective. This pulled the objective from my tracker, but left the achievement in my tracker. Going then to the achievement page, I found that the Javaal Fleet Action Mastery achievement was now listed as Incomplete. This did allow me to toggle its tracking and remove it from my tracker, but I'm now stuck with no way of completing this achievement, since all of its objectives are already finished.
So the current state of my achievements are this: Javaal Fleet Action Mastery's objectives are completed, but the achievement itself is incompleted. And for Saleucami Fleet Action Mastery, the achievement is completed, but it is still in my tracker.
Please, please sort this out. I've spent the last few patches trying desperately to get these completed achievements out of my tracker, and now I finally succeed, only to lose credit for it and be unable to get it back. I am not very pleased.