Only watch the heater that was set

Marlin/Marlin_main.cpp

     #endif
 
     #ifdef WATCH_TEMP_PERIOD
-      start_watching_heaters();
+      start_watching_heater(target_extruder);
     #endif
   }
 }
   #endif
 
   #ifdef WATCH_TEMP_PERIOD
-    start_watching_heaters();
+    start_watching_heater(target_extruder);
   #endif
 
   millis_t temp_ms = millis();

Marlin/temperature.cpp

    * their target temperature by a configurable margin.
    * This is called when the temperature is set. (M104, M109)
    */
-  void start_watching_heaters() {
-      millis_t ms = millis() + WATCH_TEMP_PERIOD;
-      for (int e = 0; e < EXTRUDERS; e++) {
-        if (degHotend(e) < degTargetHotend(e) - (WATCH_TEMP_INCREASE * 2)) {
-          watch_target_temp[e] = degHotend(e) + WATCH_TEMP_INCREASE;
-          watch_heater_next_ms[e] = ms;
-        }
-        else
-          watch_heater_next_ms[e] = 0;
-      }
+  void start_watching_heater(int e) {
+    millis_t ms = millis() + WATCH_TEMP_PERIOD;
+    if (degHotend(e) < degTargetHotend(e) - (WATCH_TEMP_INCREASE * 2)) {
+      watch_target_temp[e] = degHotend(e) + WATCH_TEMP_INCREASE;
+      watch_heater_next_ms[e] = ms;
+    }
+    else
+      watch_heater_next_ms[e] = 0;
   }
 #endif
 

Marlin/temperature.h

 void checkExtruderAutoFans();
 
 #ifdef WATCH_TEMP_PERIOD
-  void start_watching_heaters();
+  void start_watching_heater(int e=0);
 #endif
 
 FORCE_INLINE void autotempShutdown() {

Marlin/ultralcd.cpp

   fanSpeed = fan;
   lcd_return_to_status();
   #ifdef WATCH_TEMP_PERIOD
-    start_watching_heaters();
+    if (endnum >= 0) start_watching_heater(endnum);
   #endif
 }
 void lcd_preheat_pla0() { _lcd_preheat(0, plaPreheatHotendTemp, plaPreheatHPBTemp, plaPreheatFanSpeed); }