It's not affecting my game play but I figured I should still bring it up incase it messes something else up. Anyways the lost grove looks like this:

The fest zone had weird lines last time near the end too...

I'm on a chrome browser, and when I zoom out inside the browser, it gives me straight lines but vertically, probably due to the scaling of the game and how the browser compensates.
Maybe yours could be fixed similarly by zooming out?

Unfortunately, the vast majority of pages on Sylestia use fixed pixel width for stuff instead of percentages. Zooming in and out can (and most likely will) cause things like this to happen.

I believe the background to the explore area is solid black. So what you're seeing is essentially gaps in the tiles b/c the zoomed percentage isn't evenly lining up with the fixed pixel width tiles. Playing at 100% zoom should resolve the issue.