Temperature profile of the two phases system in NVT

Please post to the list, not to me.

If I understand, you thermostatted at 90K and the system
went to 90K (both parts). They you turned the thermostat
off and the temperature drifted. Especially close to the surface.

Maybe the Ar crystallizes on the surface. I suggest you viz
or otherwise examine the Ar near the surface. I doubt
this is a LAMMPS issue.