Hello, Archon!

The reported case cannot be classified as a bug since this is a part of the regular gameplay. 

Your total power can decrease even if you don't lose troops. Please refer to this article, it can help you to solve the issue. Meanwhile, I have moved your topic to the Game Discussion thread.

Reasons for troops stats decrease  can be one or more of these listed in that article that Agent Pavel linked, which are as follows: 

Did you extracted Elixirs from the Cache in the Sanctum of Asclepius?

You have swapped or removed some General's Equipment Items.?

Did your   Defense and/or Offense boost expired?

Did your Dominion Status has deactivated.?

Did action time of the Phylarch has expired?

What is not listed above: If coalition capital was downgraded then capital bonuses are also lower which also decreased your units power. Or if you joined different coalition and you are on 7 days scholar trial.