diff --git a/crates/polars-plan/src/logical_plan/functions/mod.rs b/crates/polars-plan/src/logical_plan/functions/mod.rs
index 13c8b4be3d6e..8f4866ddb73b 100644
--- a/crates/polars-plan/src/logical_plan/functions/mod.rs
+++ b/crates/polars-plan/src/logical_plan/functions/mod.rs
@@ -9,10 +9,12 @@ mod schema;
 use std::borrow::Cow;
 use std::fmt::{Debug, Display, Formatter};
 use std::hash::{Hash, Hasher};
+use std::ops::Deref;
 use std::path::PathBuf;
 use std::sync::{Arc, Mutex};
 
 use polars_core::prelude::*;
+use schema::CachedSchema;
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
 use smartstring::alias::String as SmartString;
@@ -23,8 +25,6 @@ use crate::dsl::python_udf::PythonFunction;
 use crate::logical_plan::functions::merge_sorted::merge_sorted;
 use crate::prelude::*;
 
-type CachedSchema = Arc<Mutex<Option<SchemaRef>>>;
-
 #[derive(Clone)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub enum FunctionNode {
@@ -96,6 +96,7 @@ pub enum FunctionNode {
     RowIndex {
         name: Arc<str>,
         // Might be cached.
+        #[cfg_attr(feature = "serde", serde(skip))]
         schema: CachedSchema,
         offset: Option<IdxSize>,
     },
diff --git a/crates/polars-plan/src/logical_plan/functions/schema.rs b/crates/polars-plan/src/logical_plan/functions/schema.rs
index a722e48887a4..9bcdcb0d8643 100644
--- a/crates/polars-plan/src/logical_plan/functions/schema.rs
+++ b/crates/polars-plan/src/logical_plan/functions/schema.rs
@@ -108,3 +108,29 @@ fn row_index_schema(
     *guard = Some(schema_ref.clone());
     schema_ref
 }
+
+// We don't use an `Arc<Mutex>` because caches should live in different query plans.
+// For that reason we have a specialized deep clone.
+#[derive(Default)]
+pub struct CachedSchema(Mutex<Option<SchemaRef>>);
+
+impl AsRef<Mutex<Option<SchemaRef>>> for CachedSchema {
+    fn as_ref(&self) -> &Mutex<Option<SchemaRef>> {
+        &self.0
+    }
+}
+
+impl Deref for CachedSchema {
+    type Target = Mutex<Option<SchemaRef>>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl Clone for CachedSchema {
+    fn clone(&self) -> Self {
+        let inner = self.0.lock().unwrap();
+        Self(Mutex::new(inner.clone()))
+    }
+}
diff --git a/crates/polars-plan/src/logical_plan/optimizer/projection_pushdown/functions/mod.rs b/crates/polars-plan/src/logical_plan/optimizer/projection_pushdown/functions/mod.rs
index f0b71e3ff42f..d7faf2526a9a 100644
--- a/crates/polars-plan/src/logical_plan/optimizer/projection_pushdown/functions/mod.rs
+++ b/crates/polars-plan/src/logical_plan/optimizer/projection_pushdown/functions/mod.rs
@@ -79,10 +79,6 @@ pub(super) fn process_functions(
             )
         },
         _ => {
-            let lp = IR::MapFunction {
-                input,
-                function: function.clone(),
-            };
             if function.allow_projection_pd() && !acc_projections.is_empty() {
                 let original_acc_projection_len = acc_projections.len();
 
@@ -109,6 +105,10 @@ pub(super) fn process_functions(
 
                 // Remove the cached schema
                 function.clear_cached_schema();
+                let lp = IR::MapFunction {
+                    input,
+                    function: function.clone(),
+                };
 
                 if local_projections.is_empty() {
                     Ok(lp)
@@ -127,6 +127,10 @@ pub(super) fn process_functions(
                     }
                 }
             } else {
+                let lp = IR::MapFunction {
+                    input,
+                    function: function.clone(),
+                };
                 // restart projection pushdown
                 proj_pd.no_pushdown_restart_opt(
                     lp,
diff --git a/py-polars/tests/unit/test_projections.py b/py-polars/tests/unit/test_projections.py
index 8035eaa02441..ae9dae78e6f2 100644
--- a/py-polars/tests/unit/test_projections.py
+++ b/py-polars/tests/unit/test_projections.py
@@ -408,3 +408,14 @@ def test_projection_drop_with_series_lit_14382() -> None:
         "b_name": ["b"],
         "b_old": [7],
     }
+
+
+def test_cached_schema_15651() -> None:
+    q = pl.LazyFrame({"col1": [1], "col2": [2], "col3": [3]})
+    q = q.with_row_index()
+    q = q.filter(~pl.col("col1").is_null())
+    # create a subplan diverging from q
+    _ = q.select(pl.len()).collect(projection_pushdown=True)
+
+    # ensure that q's "cached" columns are still correct
+    assert q.columns == q.collect().columns