Unexpected temperature increase at the bottom grid

Hello everyone,

I am simulating the evaporation process in the vadose zone (1-D) using EOS3 (via PetraSim).
When I activate molecular diffusion, I notice an unexpected temperature increase at the bottom grid, even though no heat source is applied there. This unexpected behavior has me quite puzzled.

I've attached all relevant input files as well as output files.
I would greatly appreciate any insights or suggestions on how to resolve this issue.

Thank you in advance for your help!