🐛 Fix AUTOTEMP bug (thermal runaway) (#23025)

Regression from 9823a37

Marlin/Configuration_adv.h

  */
 #define AUTOTEMP
 #if ENABLED(AUTOTEMP)
-  #define AUTOTEMP_OLDWEIGHT    0.98
+  #define AUTOTEMP_OLDWEIGHT    0.98  // Factor used to weight previous readings (0.0 < value < 1.0)
   // Turn on AUTOTEMP on M104/M109 by default using proportions set here
   //#define AUTOTEMP_PROPORTIONAL
   #if ENABLED(AUTOTEMP_PROPORTIONAL)

Marlin/src/module/planner.cpp

 
     float t = autotemp_min + high * autotemp_factor;
     LIMIT(t, autotemp_min, autotemp_max);
-    if (t < oldt) t *= (1.0f - (AUTOTEMP_OLDWEIGHT)) + oldt * (AUTOTEMP_OLDWEIGHT);
+    if (t < oldt) t = t * (1.0f - (AUTOTEMP_OLDWEIGHT)) + oldt * (AUTOTEMP_OLDWEIGHT);
     oldt = t;
     thermalManager.setTargetHotend(t, active_extruder);
   }