Fixed #7115

core/src/mindustry/world/blocks/units/UnitAssembler.java

@@ -228,6 +228,7 @@ public class UnitAssembler extends PayloadBlock{
         public float progress, warmup, droneWarmup, powerWarmup, sameTypeWarmup;
         public float invalidWarmup = 0f;
         public int currentTier = 0;
+        public int lastTier = -2;
         public boolean wasOccupied = false;
 
         public float droneProgress, totalDroneProgress;
@@ -275,10 +276,8 @@ public class UnitAssembler extends PayloadBlock{
                     break;
                 }
             }
-            if(currentTier != max){
-                progress=0;
-                currentTier = max;
-            }
+
+            currentTier = max;
         }
 
         public UnitType unit(){
@@ -340,6 +339,15 @@ public class UnitAssembler extends PayloadBlock{
                 readUnits.clear();
             }
 
+            if(lastTier != currentTier){
+                if(lastTier >= 0f){
+                    progress = 0f;
+                }
+
+                lastTier =
+                    lastTier == -2 ? -1 : currentTier;
+            }
+
             //read newly synced drones on client end
             if(units.size < dronesCreated && whenSyncedUnits.size > 0){
                 whenSyncedUnits.each(id -> {