UPSERT using the wrong index, unable to provide an indexHint

My Environment

ArangoDB Version: 3.7.11
Storage Engine: RocksDB
Deployment Mode: Cluster
Deployment Strategy: Kubernetes
Configuration: networkAttachedVolumes: true on the ArangoDeployment, --server.statistics=true as extra arg
Infrastructure: own
Operating System: Ubuntu 20.04.2
Total RAM in your machine: 3 nodes, all 64GB
Disks in use: 3 nodes, all SSD 512GB
Used Package: Docker - official Docker library (arangodb/arangodb:3.7.11)

Affected feature:
UPSERT index usage

Size of your Dataset on disk: 20GB

Replication Factor & Number of Shards (Cluster only):
all collections have replicationFactor: 3, writeConcern: 2 Number of shards: 1

Problem:

I have 2 identical edge collections (same setting, same indexes). The same UPSERT query:

UPSERT {
  _from: 'a',
  _to: 'b'
}
    INSERT {}
    UPDATE {}
    IN collection
    RETURN {}

uses the correct (best) index for the first collection but uses the worse one for the second collection (see image above)

I thought maybe I could provide an indexHint but that doesn't appear to be possible in UPSERT and probably should be.

Expected result:

either