I realize its a little late for this to be of any value for the STRIKE Global Event but in case its not fixed by the next Global Event its important to know that the games matchmaking process ignores what mission modifier you've selected if you have either not selected either modifier (preemptive strike or Tactical Strike) or if you have selected one of the 2 mission modifiers and then switched back to vanilla strike to indicate you are seeking to run the mission with NO MODIFIERS enabled.
EXAMPLE:
Select any mission that offers the 2 mission modifiers Preemptive Strike and Tactical Strike. Select your difficulty level (i.e. Challenging) and in the next step you can either not select a mission modifier or select one of the mission modifiers and then select Strike to explicitly indicate you want to run the mission with NO MODIFIERS active, hit Start
Matchmaking and when you are matched into a group the algorithm ignores your selection of no mission modifier and you will find yourself in a group running either preemptive strike or tactical strike.
This does not appear to be the case if you do select one of the mission modifiers at least that was the case in what little testing I was able to do on PS4 before the Global Event was over.
I don't know if this is by design (that your mission modifier choice is ignored) or if its a bug but this has been reported as a bug.
I would agree if it weren't for the fact that I was being matched within a few seconds every time I tested this with no modifier selected but if I selected Preemptive Strike or Tactical Strike it could take up to 20-30 secs to match make. That tells me its ignoring my selection of no mission modifier else it would take longer to match make.'It probably checks for Mission difficulty first and than checks for your modifier. If none are available if places you in the first available map'.