Thermosiphon inlet and outlet temperature issues

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In the debutanizer tower of the reforming unit, the bottom of the tower is a horizontal thermosiphon reboiler. The process medium goes through the shell side and the heating steam goes through the tube side. What I want to ask is, usually under normal process conditions, the temperature of the material returning to the tower from the reboiler is 123°C, the temperature of the material entering the reboiler from the column still is 126°C, and the tower pressure is controlled at about 1Mpa. The main thing I don’t understand is why the temperature of the material after being heated is 2~3°C lower than before being heated. There may be an enthalpy issue here, but I just don't understand it very well. I would like you to give me an authoritative explanation. Also, the heated material should be a gas-liquid mixture, right? ! I don't know if there is more gas or liquid. Thank you.