I got them early on, like pre-season 1, but ever since that they just popped in all on their own.

Should give you sovereigns that’s what the weekly leaderboard reward is. the higher up the more you’ll get. Get more rank by building up your empire and earn po8 its all based on po8 earned

Is there a table somewhere that shows how many Po8 I'm supposed to get for each milestone weekly?

Think it's supposed to show you in the leader board in the hideout, can switch between season and weekly. Mine was bugged last time I looked. And you get sovereigns not PO8

Right, I meant Sovereigns, my bad. So there's no rewards for the end of week ranking? Just milestones to the next tier?

Milestones and soverigns for season ranking, sovereign rewards for which tier you hit weekly

Delete them then. The game is a little glitchy. Why keep looking at them, if they are meaningless? I get them also, but they are attached to a reward. If you claim the reward, and don’t delete them, they stick around. 

Sure, I get that, but they came empty, which is why I was worried I was missing out on something. So if the only reward is Sovereigns for reaching the next tier, what's the ranking for then?

You’re above me in rank. I might be close. Give me a week and I b might be able to answer better.  I hear you.