Not sure if you're losing your mind or not - but it's not uncommon for two different instruments to read differently if placed in the same temperature bath.
Are you physically taking the measurement in the same place with both devices ?
Have you done any recalibration on the digital ?
Do you have a mercury thermometer ? You could take some tank water in a cup, and compare the digital to the mercury one.
Could be one is defective, or the RTD/Thermocouple/Thermistor is bad.
You'll have to run a test to see which one's correct or closer to being correct. Or return to manufacturer.