List subscript with an inline aggregate index may return null #4100

ArcadeDB version
Observed on Docker images:

• arcadedata/arcadedb:latest
• arcadedata/arcadedb:26.3.2
• arcadedata/arcadedb:26.4.1-SNAPSHOT
• arcadedata/arcadedb:26.4.2

Environment

• Host OS: Windows 10
• Architecture: x86_64
• Deployment: Docker
• ArcadeDB endpoint: HTTP /api/v1/command/arcade
• Request mode matches ArcadeDB Studio:
  • language: opencypher
  • serializer: studio
• Differential comparison target: Neo4j Docker neo4j:latest

Describe the bug
ArcadeDB may return null when a list subscript uses an inline aggregate expression as the index, even though:

• the same aggregate expression evaluates correctly on its own
• the same list subscript works when the index is first materialized in WITH

In the minimized repro below, Neo4j returns the first score (85).
ArcadeDB returns null.

To Reproduce

Setup:

CREATE (:Person {name:'Alice', scores:[85,92,78]});

Query:

MATCH (p:Person {name:'Alice'})
OPTIONAL MATCH (p)-[:HAS_FRIEND]->(f:Person)
WITH p, collect(f) AS friends
RETURN p AS person,
       p.scores[toInteger(avg(size(friends)))] AS selectedScore;

Expected behavior
With no matching HAS_FRIEND relationships:

• collect(f) becomes []
• size(friends) is 0
• avg(size(friends)) is 0
• toInteger(...) is 0

So the query should return the first score:

85

Observed Neo4j result:

(:Person {name: "Alice", scores: [85, 92, 78]}), 85

Actual behavior
ArcadeDB returns the person correctly, but selectedScore becomes null:

(:Person {name: "Alice", scores: [85, 92, 78]}), null

Control query 1
If the same index is computed first and then reused, ArcadeDB behaves correctly:

MATCH (p:Person {name:'Alice'})
OPTIONAL MATCH (p)-[:HAS_FRIEND]->(f:Person)
WITH p, collect(f) AS friends
WITH p, toInteger(avg(size(friends))) AS idx
RETURN p AS person,
       p.scores[idx] AS selectedScore,
       idx;

Observed result on both Neo4j and ArcadeDB:

(:Person {name: "Alice", scores: [85, 92, 78]}), 85, 0

So the aggregate expression itself is not the problem.

Control query 2
If the subscript uses a non-aggregate index, ArcadeDB also behaves correctly:

MATCH (p:Person {name:'Alice'})
OPTIONAL MATCH (p)-[:HAS_FRIEND]->(f:Person)
WITH p, collect(f) AS friends
RETURN p AS person,
       p.scores[size(friends)] AS selectedScore;

Observed result on both Neo4j and ArcadeDB:

(:Person {name: "Alice", scores: [85, 92, 78]}), 85

This makes the boundary clearer: the failure is tied to using the aggregate expression inline inside the list subscript.