I quite agree. It's a strange thing that the dslreports.com Tweak tool turns my MTU of 1430 (all my hosts and router are set to this value) into 1390. I'm mystified but it's always done this. Anyway, as long as it doesn't increase the MTU, it won't cause packet fragmentation so no harm done.
You seem to share my view that congestion is responsible for packet loss. What else could it be?
I am going to use Tweak during the Ukraine v England match to try to determine what effect the enormous video stream traffic might be having on my internet experience at that time.