Water barrels fill even when not connected to anything, so you would not start with them empty anyway. I would suggest waiting until water shuts off normally, then we can check if there is an issue with plumbing the sinks to the barrels. You can send the save once it happens if you run into issues with it.

The problem here is that the water in the world has not shut off yet, so yes, all sinks will work no matter where they're placed. And that is not my issue. My issue is specifically plumbing to placed rain barrels on the ground floor, after I've moved around plumbing fixtures to suit the rain barrel positioning.   All of this was preparing for when the water in the world eventually does shut off, so that I don't start off with empty rain barrels at the point of water shutting off.   So that's when I noticed that some of the sinks (at location 1 & 3), after being moved, don't show the plumb option to the rain barrels (it should be an additional option when world's water is still on).   And if it's not recommended to turn off the water (via Sandbox Settings etc.) just to test, then I can't think of how else to convey what's happening in-game.

Sorry I am still not sure I understand, you can see even in your own screenshot that the water works fine in all the sinks, those that it does not work in you can plumb them, meaning you have water in all the sinks. Changing sandbox options mid-game is not really recommended, so I would not consider doing that for testing. Do any of your sinks not get water? Because in the save you sent all of them either have water currently or can be plumbed, meaning they all work as intended. When I move the sinks to the positions you specified that are not plumbable, I get the plumbing option as shown in my screenshots, so the water again works correctly.

Version: 42.7.0 No Mods New Save   1. Have a water bottle in your player inventory that has water in it. It must not be full but it has to have water. Maybe other containers work too. 2. Have a gas can with gasoline on the floor. That one can be full or partial. 3. Right click on the can of gas.   STACK TRACE ----------------------------------------- function: doFillFluidMenu -- file: ISWorldObjectContextMenu.lua line # 4014 | Vanilla function: doFluidContainerMenu -- file: ISWorldObjectContextMenu.lua line # 5890 | Vanilla function: createMenu -- file: ISWorldObjectContextMenu.lua line # 1012 | Vanilla function: createMenu -- file: ISMenuContextWorld.lua line # 50 | Vanilla function: createWorldMenu -- file: ISContextManager.lua line # 28 | Vanilla function: doRClick -- file: ISObjectClickHandler.lua line # 60 | Vanilla function: onObjectRightMouseButtonUp -- file: ISObjectClickHandler.lua line # 426 | Vanilla ERROR: General      f:248, t:1746527085846> ExceptionLogger.logException> Exception thrown     java.lang.RuntimeException: attempted index: getName of non-table: null at KahluaThread.tableget(KahluaThread.java:1667).     Stack trace:         se.krka.kahlua.vm.KahluaThread.tableget(KahluaThread.java:1667)         se.krka.kahlua.vm.KahluaThread.luaMainloop(KahluaThread.java:624)         se.krka.kahlua.vm.KahluaThread.call(KahluaThread.java:173)         se.krka.kahlua.vm.KahluaThread.pcall(KahluaThread.java:1963)         se.krka.kahlua.vm.KahluaThread.pcallvoid(KahluaThread.java:1790)         se.krka.kahlua.integration.LuaCaller.pcallvoid(LuaCaller.java:66)         se.krka.kahlua.integration.LuaCaller.protectedCallVoid(LuaCaller.java:139)         zombie.Lua.Event.trigger(Event.java:72)         zombie.Lua.LuaEventManager.triggerEvent(LuaEventManager.java:410)         zombie.ui.UIManager.updateMouseButtons(UIManager.java:953)         zombie.ui.UIManager.update(UIManager.java:706)         zombie.GameWindow.logic(GameWindow.java:332)         zombie.GameWindow.frameStep(GameWindow.java:928)         zombie.GameWindow.run_ez(GameWindow.java:821)         zombie.GameWindow.mainThread(GameWindow.java:619)         java.base/java.lang.Thread.run(Unknown Source)  

In your screenshot (below) there is no "Plumb Sink" option, even though it's positioned below a rain barrel installed above it. So it's sourcing water from the water mains, since the water hasn't shut off yet.       But I just tested the same save file I sent, removed all mods (except the Grapeseed map mod - since that prevents the world from loading if removed), and I'm still not getting an option to plumb items to the rain barrels at locations 1 & 3, when the plumbing fixtures are moved around. This is the same issue happening at my base location.   Location 1 (Moved Kitchen Sink - No Plumb Option To Connect Barrels)   Location 3 (Moved Bathroom Sink - No Plumb Option To Connect Barrels)   So the option with the red arrow (below) doesn't pop up, which allows plumbing to the rain barrels from the balcony above, and removes the item from the water mains. The option at the green arrow is related to the items connected to the water mains (currently), and the item has water that way.   Another test you can do from my original save file: Remove all mods, load the save, open sandbox settings and turn of the water supply in the world. Then this should force the plumbing fixutres to look for any nearby rain barrels only. I'm sure this will result in no option popping up to connect these pluming fixtures to the rain barrels.   I hope this is much clearer.