Welcome to iMore! I wouldn't say it's normal but this isn't the first I'm hearing about this and it doesn't appear to do any harm. If you're really worried you can always contact Apple Support or bring your iPad Pro into an Apple Store and have a Genius look at it.
Interesting, I've come across a lot of site transformers that do this, usually when they're a bit old and battered, I think you should talk to Apple about this, doesn't sound right to me.