Generally it doesn't actually tell you why.

lastwake will be blank and the power report will just tell you it changed state but give no reason.

I found the same thing when I was trying to figure out why an Intel NUC7i5 keeps waking up for no reason.

One would hope that powercfg /lastwake provides some information, but it doesn't.

There is a big difference between how Window should work in theory and what happens in practice.