The TRReset state is not needed with fall-through

Marlin/temperature.cpp

 
     // If the target temperature changes, restart
     if (tr_target_temperature[heater_index] != target_temperature)
-      *state = TRReset;
+      *state = TRInactive;
 
     switch (*state) {
-      case TRReset:
-        *timer = 0;
-        *state = TRInactive;
       // Inactive state waits for a target temperature to be set
       case TRInactive:
         if (target_temperature <= 0) break;

Marlin/temperature.h

 
     #if ENABLED(THERMAL_PROTECTION_HOTENDS) || HAS_THERMALLY_PROTECTED_BED
 
-      typedef enum TRState { TRReset, TRInactive, TRFirstHeating, TRStable, TRRunaway } TRstate;
+      typedef enum TRState { TRInactive, TRFirstHeating, TRStable, TRRunaway } TRstate;
 
       void thermal_runaway_protection(TRState* state, millis_t* timer, float temperature, float target_temperature, int heater_id, int period_seconds, int hysteresis_degc);
 
       #if ENABLED(THERMAL_PROTECTION_HOTENDS)
-        TRState thermal_runaway_state_machine[EXTRUDERS] = { TRReset };
+        TRState thermal_runaway_state_machine[EXTRUDERS] = { TRInactive };
         millis_t thermal_runaway_timer[EXTRUDERS] = { 0 };
       #endif
 
       #if HAS_THERMALLY_PROTECTED_BED
-        TRState thermal_runaway_bed_state_machine = TRReset;
+        TRState thermal_runaway_bed_state_machine = TRInactive;
         millis_t thermal_runaway_bed_timer;
       #endif