There *should* be a back flow valve between the city water and the fresh water tank. Often it's integrated with the water pump. AS changes this and that from time to time. One can never say "always" about anything
Best guess is that you have some crud in the valve. If that happens, it sticks open and the tank backfills. Running it a couple times in the "forward" direction may clear it. In extreme cases (lots of stuff in the tank), doing a fill / drain on the fresh water tank first might be needed.
There also is a strainer on most water pumps. If you have a crud problem, it's likely that it needs to be cleaned out.
Stuff does break from time to time, so that could be the issue as well. The good news there is that you have a warranty and a good dealer just up the road.
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