Look at the mix when the melt hits its slush zone, where it's part solid part liquid. Lead's melting point is just over 620°F, so pure lead should be in that range and alloys with tin and antimony should be lower. Wheel weights, for example, melt at under 500°F. So, if the thermometer still reads 900 when the metal is hardening, then you definitely have a problem with the thermometer.
Be aware that the numbers on the Lee pots are not very precise. The thermostat is in the box and not tied directly to the pot, and the one I have wanders about 50° up an down no mater where I set it. I finally made my own temperature control for it.
I and others have used those thermometers for years without problems, but there is no guarantee yours wasn't dropped or something. Conventional thermometers are not typically rugged, so they have to be handled with care.
I use electronic thermocouple thermometers these days. They are less delicate. I got some on sale on closeout for $20 one time that work fine for casting (good withing about 5°). More accuracy costs more, but keep your eye out. Make sure the thermocouple itself is rated for the temperature range you need. Types J or K in glass woven insulation are good.
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