Hello, water will evaporate from the bowl over 5 days in most cases, sooner if there are pets drinking water from the toilet... is that a possibility here?
You can do a test, dip a few cups of water from the bowl from that toilet and a few others tell me if any of the good ones, or this bad one refill automatically. If not, evaporation is the most likely cause of the problem
I am a retired mechanical engineer and mechanical/ electrical/ refrigeration and boiler contractor ... with 56 years in this business.
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