Series Connection of thermal networks

PLECS
2

My understanding is that due to the nature of the Foster network, any changes to the system, such as to arbitrarily extend the network out from the junction, requires recalculation of all values. So I think this may lead to the discrepancy you are seeing in the synthesized Foster network. This resource may also be helpful for you: https://www.infineon.com/dgdl/Infineon-Thermal_equivalent_circuit_models-ApplicationNotes-v01_02-EN.pdf?fileId=db3a30431a5c32f2011aa65358394dd2