diff --git a/crates/polars-expr/src/expressions/window.rs b/crates/polars-expr/src/expressions/window.rs
index 5a455cf5932b..b47d1744f662 100644
--- a/crates/polars-expr/src/expressions/window.rs
+++ b/crates/polars-expr/src/expressions/window.rs
@@ -315,7 +315,6 @@ impl WindowExpr {
     fn determine_map_strategy(
         &self,
         agg_state: &AggState,
-        sorted_keys: bool,
         gb: &GroupBy,
     ) -> PolarsResult<MapStrategy> {
         match (self.mapping, agg_state) {
@@ -334,13 +333,8 @@ impl WindowExpr {
             // no explicit aggregations, map over the groups
             //`(col("x").sum() * col("y")).over("groups")`
             (WindowMapping::GroupsToRows, AggState::AggregatedList(_)) => {
-                if sorted_keys {
-                    if let GroupsProxy::Idx(g) = gb.get_groups() {
-                        debug_assert!(g.is_sorted_flag())
-                    }
-                    // GroupsProxy::Slice is always sorted
-
-                    // Note that group columns must be sorted for this to make sense!!!
+                if let GroupsProxy::Slice { .. } = gb.get_groups() {
+                    // Result can be directly exploded if the input was sorted.
                     Ok(MapStrategy::Explode)
                 } else {
                     Ok(MapStrategy::Map)
@@ -516,7 +510,7 @@ impl PhysicalExpr for WindowExpr {
         let mut ac = self.run_aggregation(df, state, &gb)?;
 
         use MapStrategy::*;
-        match self.determine_map_strategy(ac.agg_state(), sorted_keys, &gb)? {
+        match self.determine_map_strategy(ac.agg_state(), &gb)? {
             Nothing => {
                 let mut out = ac.flat_naive().into_owned();
 
diff --git a/crates/polars/tests/it/lazy/expressions/window.rs b/crates/polars/tests/it/lazy/expressions/window.rs
index 21d8a3d26bf7..fb52ac3810bb 100644
--- a/crates/polars/tests/it/lazy/expressions/window.rs
+++ b/crates/polars/tests/it/lazy/expressions/window.rs
@@ -150,9 +150,7 @@ fn test_sort_by_in_groups() -> PolarsResult<()> {
             col("cars"),
             col("A")
                 .sort_by([col("B")], SortMultipleOptions::default())
-                .implode()
                 .over([col("cars")])
-                .explode()
                 .alias("sorted_A_by_B"),
         ])
         .collect()?;
diff --git a/py-polars/tests/unit/operations/test_window.py b/py-polars/tests/unit/operations/test_window.py
index 8171fd5f9b03..69b9cd8f0e55 100644
--- a/py-polars/tests/unit/operations/test_window.py
+++ b/py-polars/tests/unit/operations/test_window.py
@@ -518,3 +518,21 @@ def test_lit_window_broadcast() -> None:
     assert pl.DataFrame({"a": [1, 1, 2]}).select(pl.lit(0).over("a").alias("a"))[
         "a"
     ].to_list() == [0, 0, 0]
+
+
+def test_order_by_sorted_keys_18943() -> None:
+    df = pl.DataFrame(
+        {
+            "g": [1, 1, 1, 1],
+            "t": [4, 3, 2, 1],
+            "x": [10, 20, 30, 40],
+        }
+    )
+
+    expect = pl.DataFrame({"x": [100, 90, 70, 40]})
+
+    out = df.select(pl.col("x").cum_sum().over("g", order_by="t"))
+    assert_frame_equal(out, expect)
+
+    out = df.set_sorted("g").select(pl.col("x").cum_sum().over("g", order_by="t"))
+    assert_frame_equal(out, expect)