I've come across this a few times now. Are they just bugged out or is this witch craft They don't move. They don't burn up in the day time. They don't harm you. They just stand there frozen. Maybe they just want a hug.
Might be true. When it happens, try turning off mods and disabling your resource pack(s). Or else it might be a zombie lagging out due to a slow internet connection. You could try pinging (/ping) when the glitch happens again to see if that's the case (the lower the better).
I use WorldDownloader sometimes so I can build my farms in creative first to make sure I like them. I though maybe that was what was causing this.