Engine dies when brake is released. - John Deere 26HP Hydrostatic 54" Riding Mower BG20559

check the safety switches

z425 shows ignition safety logic is as follows;

Engine RUN if Seat Occupied or Brake ON and PTO OFF.

Symptoms imply seat occupied circuit is OPEN. Check seat switch/wiring.

Id say that even though you say the operator seat switch "looks" ok the symptom you describe is a perfect match to a defective seat switch and or wiring

This sounds like the seat switch. Keep in mind newer model seat switches cannot be jumped or disconnected.

check the seat safety switch first...

check if the switch for the blades or the brake is not releasing.

there is a saftey switch on the deck that has shorted out need to replace

check switch on brake

First check the seat switch,make sure it is plugged in tightly, these plugs have a safety tab built into them that will cut the engine off if you unhook it,sometimes the plug latch pulls out slightly and this will cut the engine, There are two other switches in the system one on the foot clutch and on on the mower clutch, if you follow the white wire from you ignition switch it will lead you to both of them,check those connections also.

I would check the wires and clean the connectors and make sure seat switch is working. If that is not working, it's like getting off the seat without holding the brake/clutch pedal down or getting off seat with blades engaged. Both will cause engine to die.