Why am I getting incorrect results via DC test performed in Matlab/Simulink?
This version of the equivalent circuit of an induction motor is the most complete version that is commonly used. You can see that when a DC voltage is applied between L and N, all of the inductors become short circuits and what is left is simply Rs and the DC source. The transformer and the components in the secondary circuit essentially disappear.

Diagram adapted from Malcom Barnes &quot;Practical Variable Speed Drives and Power Electronics&quot;
In the simulation, I would expect the same, except that there is Rsa, Rsb and Rsc connected as shown below. The resistance of the connected combination of three resistors would be 1.5 times an individual resistor value, so the multiplier in the simulation, must be 2/3.

The resistance test can be performed by connecting to two terminals with no connection to the third terminal. Note that you can not determine the true resistance unless you know whether the motor is connected wye or delta. If you don't know the connection, assume wye. The equivalent circuit is based on the wye connection, so the wye equivalent is used for analysis regardless of the actual connection. IEEE Std 112-2017 specifies measuring the terminal to terminal resistance.