iMovie eating battery while connected with power adaptor

Hello

One month ago i needed a fast way to do a small video montage.

Normally i use FCP at work, but for this montage at home i have chosen iMovie and start to work with it.

All went fine until i did the (mp4) export.

It was the first time in many years that i heard the fans of my MacBook Pro screaming.

I was ok with that but then i noticed that iMovie was draining my battery very fast while the MacBook Pro was connected with the power adaptor.

I have never seen something like that in +-20 years working with Mac computers!

The battery/power symbol in the menu bar was showing that the power adaptor was connected and giving power, meanwhile the percentage from the battery whas going down???

See attachment from Activity Monitor, taken while this was happening.

You see that there is shown: "Battery is not charging", but it was.

How is this possible and what does this with the battery?

How can a software like that have such bad performance?

Best regards