1990 300SEL 49 state model does not send 12v to oxygen sensor heater power wire

Backstory: CEL, check duty cycle across pins 2 and 3 on X11 connector, get 50%. Disconnect oxygen sensor signal wire in passenger footwell. Monitor signal directly. Not getting good signal. Check voltage from harness, get ~ 0.3 volt from control unit with voltage falling over period of several seconds. Neglect to check sensor heater power from control side.

Assume old oxygen sensor is tired. Install new oxygen sensor. Same result, no signal from oxygen sensor and car fails to go into closed loop even after loading engine to warm sensor. CEL, 50% duty cycle reading across pins 2 and 3 at X11 connector.

Pull sensor, verify with propane torch that sensor works. Reinstall. Check heater power. With key on, see ~3 millivolt from power side of oxygen sensor heater. Verify ground side goes to good ground.

Fabricate add-on harness to manually power oxygen sensor heater. With O2 heater plug from car's harness disconnected and manually sending 12v to sensor heater, car goes closed loop as expected shortly after starting engine. Monitoring duty cycle across pins 2 and 3 of X11 connector, get fluctuating signal between ~40-50%. No CEL.

Pull MAS and ECU. Run continuity test between O2 sensor heater power wire and pin 20 of MAS. Good continuity. Run continuity test for O2 sensor heater diagnostic between pins 18 of MAS and 3 of ECU. Good continuity. Pull cover of ECU, no obviously burned components or traces. MAS not easy to disassemble, did not disassemble.

Tested pins 21 and 10 at MAS connector, good power with ignition switched on. Turning ignition key on cycles the fuel pump. Removing the MAS with the ignition on and reinstalling it cycles the fuel pump.

With MAS removed and pin 21 (hot with ignition on) jumpered to pin 20 (out to O2 sensor heater), with ammeter hooked in series with O2 sensor heater, with ignition off no current, with ignition switched on, ~3 amps falling quickly toward 1 amp as sensor heats up.

So what would keep the MAS from powering up the O2 sensor heater but allow the car to run in otherwise normal closed loop operation if I disconnect the heater from the car's harness and power it externally?