I do have the same issue. Finished Sirius about 29 times now and after yesterdays update I am unable to get the mission again. Same for Regulus. As I imagine it will be like this for all of the others.
I did too. I think they re-up every time you complete them. And of course, you have to defeat X squadrons three times. If the mission is automatically reenable, that pleases me greatly. Now, if the explore the nebula/arm/expanse/cluster mission are the same way too, I am greatly please.