Welcome to the forums, sorry to hear about your frustration.
804 is an excellent score, congratulations for that.
Was it several months since that Capital One card had reported any balance on a statement?
Usually if all Credit Cards are reporting zero balances at the same time, one would expect a lower score. It may be that this card was "dormant" for several months and is now newly active. If it continues to report a small balance each month, so it looks consistently active, and the other cards report zeros, your score could recover from the "recently active" card situation and climb even higher than before.
