I only have two pages A links to B. Both have a preload of data. So I load A, everything is nice. Click on the link to B and voila - the new layout and everything, but the onMount from A renders components into page B. WTF? What happened?
Turns out I had a small error in onMount A, which caused the app to get confused. I don't know why or how. Just make sure you handle all errors on onMount.
Took me 4 hours today. Since problem was only in production and I couldn't reproduce it. Eff..