This happens in 18.04 too. Bug #1650076 does also, and I’m not 100 % sure if these are two different issues or the same. I was able to reproduce #1650076 with the steps listed there, but just before that I had Evince following my scroll wheel despite being unfocussed, and I had not selected anything in the PDF before that (at least not knowingly). I haven’t found the steps to reproduce this without the selection though.