Copying from one page to another and switching back causes code to be cut

When I have multiple pages open in code view  , I copy and paste from one page to the other.  When I switch back to the page i copied from the text cuts and leaves a  "-" character in its place.  Its driving me nuts as I keep loosing code if i am not very careful.

2017.5
build 9878

this the latest build i have through the creative cloud app.