Mysterious Disappearing Epics?

So, the weather finally got hospitable enough for me to go out and dart the Epics within walking distance of my house, and the second I get into range of a Sonorasaurus, it disappears!

Suffice to say, I turned around and went straight back home.

Anyone else encountering this? (I assume it’s a glitch)

They change around a particular time. I think one vanishes and it take some time for the other to appear.