The issue is with the "changelevel" command in Garry's Mod (Garry you dumb ass how did you manage to break changelevel). Normally (unusually) Garry would fix this instantly (it started like 3 weeks ago), but he doesn't know what is causing it (the "optimization" by removing, without replacing immediately, things would be a good starting point) and probably still hasn't fixed it. I restarted (crashed) the server (smash because slope crashes itself), so always extend canal or else... (this will happen again.)