Situation:

I flush the upstairs toilet. It flushes just fine.

About 3 minutes later, the other toilet downstairs does the end-part of a
flush. I can hear the valve downtsairs close(makes a reassuring "thunk"
sound).

Did a little experiment. Flushed upstairs toilet, ran downstairs and watched
the other toilet. The water level in the tank goes down a bit, and it of
course flushes back to normal level.

Any remedy for this?

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