Yeah good point, I was suspecting this could be an issue. However I went ahead since the original analogue thermostat of the boiler uses the exact same copper lead. (see picture below)
I am considering to change my setup and use a T-split on the warm-out of the boiler and immerse a probe holding the DS18B20 around 20cm / ~8in into the lower/colder part of the tank. Will do that in the next days and let you know 
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