Transfer clutch burn out is not uncommon with Subaru transmissions. Normal wear and tear, sealing ring failure or molded piston failure (especially with glycol intrusion) would be a few items on the list.
Using the same hydraulic schematic for the 4EAT Phase 2 Version 2 used in the “Seven Sisters” article (figure 2), oil from the solenoid used to stroke the transfer clutch valve goes between the face of the valve and the retaining bore plug. Should that bore plug wear around the sealing wall due to vibration, the solenoid cannot control the stroke of the valve correctly. This may prevent full line pressure from reaching the clutch.
Once the groove is made, carefully tap the plug in far enough to get the keeper installed. Over traveling the plug deep into the bore interfere with the valve’s proper position.