"plain strange" is correct and usable in written English.
It can be used in situations where something is unexpectedly or surprisingly strange. For example, "The way he acted was plain strange."
Some experiences in space are just plain strange.
New works may be exciting, amusing, annoying, engrossing or just plain strange.
A couple of the dishes may seem plain strange to some diners.
Then, there can be the just plain strange and unlikely, and all of it risked for what?
But the Big Apple nevertheless harbors dozens of bars with delicious, ambitious or just plain strange things to eat.
To contemporary audiences, it's just plain strange when a drawing-room comedy turns into a Victorian melodrama.
The manager of the moving company the Kasners used made plain how strange it was.
All this makes very plain the strange fact that our world today has more rapid communication than ever before – and yet we seem less able to see and to face the reality that's there in front of us.
Many are hard, others disarmingly modern and plain: "better days," "strange bedfellows," "sorry sight".
For these men, says Mr Philbrick, the Plains were a strange and unworldly place.
These new sights and strange plains were created by randomly stitching together satellite images and changing their photorealism through generative practices, along with adding in swooping camera movements to create the sensation of flying.
