So, what’s happening here all depends on when those games were won. To put it simply, only the games that end after the queue window expires count towards which faction gets the buff.
If you won a game after the queue window expired (as in, nobody is allowed to queue in that period anymore) and the buff didn’t go out, then yes, this is bugged.
If all these games were won BEFORE the queue window expired, then no, this is working as intended. For whatever reason, blizzard decided that the essence buff should be wiped from the server whenever a new Wintergrasp battle begins in that queue window. Insanely dumb, yes, but sadly deliberate according to their own post.
More details on the unintuitive workings of wintergrasp are in this post: