The power level decreasing I believe is just a visual bug, but yeah that's also come up for me a bunch of times.

When you go into your ninja menu and say, cultivate or awaken a ninja, one that isn't even being used in any of your teams so shouldn't have any effect on your power level in the first place; often when you close back out of the ninja menu it'll do the power calculating at the bottom of the screen saying you're dropping in power. You can leave the team you're actually using completely untouched but if you change another ninja or recruit a new ninja it'll say your power is recalculating when you close the tab even though it isn't and has no reason to. Sometimes if you upgrade ninja you actually are using it still does it when if anything you should be increasing in power. The final numbers seem to be correct though so I think it's just the wrong animation is being shown