What you were expecting to happen, and what actually happened:
I was expecting to be able to collect seals as part of progressing through PvP milestones. It looks like it was totally forgotten to migrate this part of activity related guild rewards to the new PvP system.
If you consider that it took around 20-25 battles (depending on pvp battle) to achieve Tier 1 on the old model and many more battles to get goal mark 20 on this new model, they could change this seal task to contemplate that but the amount of seals could be a little higher. Or, mantain the “x15” but increase the pvp rank battle since we have more battles to do.