My alliance was also dropped from level 6 down to level 1 with zero experience. When it happened I received the "congratulations" message for reaching level 1, then another for reaching level 6, back and forth, and eventually settled at level 1 finally.
I checked the known issues list and I do not see this, however it seems to be happening to many people, including some of the top ranked alliances if you check the top of the leaderboards.
I think the important question we are all asking is: Will the affected alliances receive the XP that they have earned, or is it lost forever and need to be earned again?
It would be nice if this issue was at least acknowledged. As it is, we don't know if we are going to be able to count on keeping the alliance XP we earn or if our alliances dropping to level 1 periodically is just going to be a thing that happens to us.