When i tried to spawn a Strider in the overworld in survival mode weird things happened as seen in the video.

That stuff with the netherack happend when i tried to leftclick the blocks and it got removed when i rightclicked it again.

After i died i couldn't move, no matter which gamemode i was in. 

I attached the video link which shows all the things i could find after spawning the Strider in the overworld.
I hope i can help.

The link to the video https://youtu.be/-x3nLz3ZWyk