Yup, the bases of 1-piece rides and also paths both lock the terrain in their immediate vicinity (often in strange ways) so you can't edit the terrain right there once those things are in place. So, you should try to do all your dirt work done prior to building anything. If you need to do more after you've got the ride and/or paths in place, you'll have to rip them out to fix the terrain.
Buildings, scenery, and the supports of tracks and paths don't cause any problems. However, if you pull a hill up under a track section until the track starts to auto-tunnel a cut in the hilltop, sometimes it's hard to lower the hill again, too.