Thank you for posting the complete models. The reason why the temperature decreases is because the conduction losses at the operating point you’re modeling are quite low compared to the switching losses and the energy flowing to ambient.
Consider the following simple circuit, where Qsw represents the device losses. After a turn-on event Tj has a step increase. If (Tj-Tamb)/Rth < Qsw then the temperature will decrease even though the device is conducting and Qsw due to conduction losses is non-zero.
I’ll also note that the IGBT thermal model has the “Convert to Cauer At Simulation Start” option unchecked in the IGBT’s Thermal Description tab. One should check this option if the device is not connected to a constant temperatures. See this previous forum post discussing issues of series connected foster networks.