Grey lines through exported photos

I shoot images in RAW - load to Lightroom on my IPAD, edit....all good.

Wheh I export these phots as a JPEG, I get strange grey lines / blocks through them on occassion. Sometimes it's 1 in 50, sometimes 1 in 5...

It's killing me. I've tried exporting as original size (right around 20mB per), and reduced all the way down to about 3 mB per photo. I've alos tried exporting / sharing multiple ways.

I can not figure out what's causing it, only that it happens when I go through these steps. 

To export on my LR mobile, I hit:

• select image
• share
• Export as...
• JPG - 60% image quality (this varies)
• hit the check mark.
• then hit save to files and select my attached hard drive

I jst did this wiht 85 images, and 5 have problems. 

See below - any help is greatly appreciated.