I could be wrong here but I don't think you understand how that log works.
It's logging everything, every little change- team changes, Ninja changes, you move a Ninja to another position that has more Life instead of Ninjutsu, that log changes.

So with that, there are a number of things that could have happened for you to decrease in power but make no mistake- you didn't lose power.
Group Summon which you mentioned to even not meeting the required Sun/Moon requirements for the stat boosts c*l alter how much power you have.

Perhaps you changed something accidentally, maybe your assist link Ninjas changed, perhaps you don't have a treasure tool equipped- any number of things could have happened for that log to show a change, but when it says -*x it doesn't mean you're constantly losing power, it's just logging a change that has happened somewhere which you can possibly revert.