diff --git a/src/include/expression_evaluator/expression_evaluator.h b/src/include/expression_evaluator/expression_evaluator.h
index dec1f28d6a..01a340d508 100644
--- a/src/include/expression_evaluator/expression_evaluator.h
+++ b/src/include/expression_evaluator/expression_evaluator.h
@@ -84,7 +84,12 @@ class ExpressionEvaluator {
 
     bool updateSelectedPos(common::SelectionVector& selVector) const {
         auto& resultSelVector = resultVector->state->getSelVector();
-        if (resultSelVector.getSelSize() > 1) {
+        // The boolean-only path below is valid only for genuinely flat result states. It must
+        // not be taken for an unflat chunk whose current selection happens to have size 1: the
+        // caller may activate the selection buffer via setToFiltered afterwards, so skipping
+        // the buffer write would expose stale positions left over from a previous batch
+        // (issue #692: a rel-property IN filter duplicated one row and dropped another).
+        if (!resultVector->state->isFlat()) {
             auto numSelectedValues = 0u;
             for (auto i = 0u; i < resultSelVector.getSelSize(); ++i) {
                 auto pos = resultSelVector[i];
diff --git a/test/test_files/issue/issue692.test b/test/test_files/issue/issue692.test
index 210c3f268b..38a48649b6 100644
--- a/test/test_files/issue/issue692.test
+++ b/test/test_files/issue/issue692.test
@@ -67,3 +67,56 @@ FileA|Function1|CALLS|0.850000
 -STATEMENT MATCH (s:Function)-[r:CodeRelation]->(t) WHERE t.id = 'Function1' AND r.type IN ['CALLS'] RETURN s.id AS src, t.id AS dst, r.type AS type, r.confidence AS conf;
 ---- 1
 Function2|Function1|CALLS|0.850000
+
+-CASE Issue692InPredicateStaleSelectionAfterCopy
+# Deterministic reproduction for github issue #692 via the bulk COPY path.
+# Shape that triggers the stale-selection bug in the no-selectFunc filter
+# fallback (updateSelectedPos): the File->Function pair holds two edges
+# between the SAME node pair (DEFINES before CALLS, so the matching row is
+# not at position 0), followed by a single-row Function->Function batch.
+# Before the fix, the IN filter returned the f0 row twice and dropped F10.
+
+-STATEMENT CREATE NODE TABLE Function(id STRING, PRIMARY KEY(id));
+---- ok
+-STATEMENT CREATE NODE TABLE File(id STRING, PRIMARY KEY(id));
+---- ok
+-STATEMENT CREATE REL TABLE CodeRelation(FROM File TO Function, FROM Function TO Function, type STRING);
+---- ok
+-STATEMENT COPY Function FROM "${LBUG_ROOT_DIRECTORY}/test/test_files/issue/issue692_fixture/function.csv" (header=false);
+---- ok
+-STATEMENT COPY File FROM "${LBUG_ROOT_DIRECTORY}/test/test_files/issue/issue692_fixture/file.csv" (header=false);
+---- ok
+-STATEMENT COPY CodeRelation FROM "${LBUG_ROOT_DIRECTORY}/test/test_files/issue/issue692_fixture/file_fn.csv" (from='File', to='Function', header=false);
+---- ok
+-STATEMENT COPY CodeRelation FROM "${LBUG_ROOT_DIRECTORY}/test/test_files/issue/issue692_fixture/fn_fn.csv" (from='Function', to='Function', header=false);
+---- ok
+
+# Ground truth: three incoming edges on F11
+-STATEMENT MATCH (c)-[r:CodeRelation]->(t {id: 'F11'}) RETURN c.id AS src, r.type AS type ORDER BY src, type;
+---- 3
+F10|CALLS
+f0|CALLS
+f0|DEFINES
+
+# Equality predicate (was already correct)
+-STATEMENT MATCH (c)-[r:CodeRelation]->(t {id: 'F11'}) WHERE r.type = 'CALLS' RETURN c.id AS src, r.type AS type ORDER BY src;
+---- 2
+F10|CALLS
+f0|CALLS
+
+# Single-element IN predicate (the bug: returned f0 twice, dropped F10)
+-STATEMENT MATCH (c)-[r:CodeRelation]->(t {id: 'F11'}) WHERE r.type IN ['CALLS'] RETURN c.id AS src, r.type AS type ORDER BY src;
+---- 2
+F10|CALLS
+f0|CALLS
+
+# list_contains spelled directly (same code path)
+-STATEMENT MATCH (c)-[r:CodeRelation]->(t {id: 'F11'}) WHERE list_contains(['CALLS'], r.type) RETURN c.id AS src, r.type AS type ORDER BY src;
+---- 2
+F10|CALLS
+f0|CALLS
+
+# Complement stays correct
+-STATEMENT MATCH (c)-[r:CodeRelation]->(t {id: 'F11'}) WHERE NOT (r.type IN ['CALLS']) RETURN c.id AS src, r.type AS type;
+---- 1
+f0|DEFINES
diff --git a/test/test_files/issue/issue692_fixture/file.csv b/test/test_files/issue/issue692_fixture/file.csv
new file mode 100644
index 0000000000..844dc808a1
--- /dev/null
+++ b/test/test_files/issue/issue692_fixture/file.csv
@@ -0,0 +1 @@
+f0
diff --git a/test/test_files/issue/issue692_fixture/file_fn.csv b/test/test_files/issue/issue692_fixture/file_fn.csv
new file mode 100644
index 0000000000..cc9de8f980
--- /dev/null
+++ b/test/test_files/issue/issue692_fixture/file_fn.csv
@@ -0,0 +1,2 @@
+f0,F11,DEFINES
+f0,F11,CALLS
diff --git a/test/test_files/issue/issue692_fixture/fn_fn.csv b/test/test_files/issue/issue692_fixture/fn_fn.csv
new file mode 100644
index 0000000000..8130515f78
--- /dev/null
+++ b/test/test_files/issue/issue692_fixture/fn_fn.csv
@@ -0,0 +1,3 @@
+F8,F9,CALLS
+F10,F11,CALLS
+F12,F13,CALLS
diff --git a/test/test_files/issue/issue692_fixture/function.csv b/test/test_files/issue/issue692_fixture/function.csv
new file mode 100644
index 0000000000..ef82f7bb49
--- /dev/null
+++ b/test/test_files/issue/issue692_fixture/function.csv
@@ -0,0 +1,6 @@
+F8
+F9
+F10
+F11
+F12
+F13