An aggregation pipeline $lookup fails even if the 2 involved collections are unencrypted. The underlying connection has csfle enabled since the same connection is used to work on the collections with the encrypted fields.
Exception received:
Exception in encryption library: Command failed with error 51204 (Location51204): 'Pipeline over an encrypted collection cannot reference additional collections.' on server localhost:27020. The full response is

{"ok": 0.0, "errmsg": "Pipeline over an encrypted collection cannot reference additional collections.", "code": 51204, "codeName": "Location51204"}

code snippet:
Bson lookup = new Document("$lookup",
new Document("from", "samplecollection1")
.append("localField", "field1")
.append("foreignField", "field2")
.append("as", "samplecollection1"));

List<Bson> filters = new ArrayList<>();
filters.add(lookup);

MongoCollection<Document> collection = encryptedClient.getDatabase(recordsDb).getCollection("samplecollection2");
AggregateIterable<Document> it = collection.aggregate(filters);

for (Document row : it)

{ System.out.println(row.toJson()); }

Note: the aggregation works fine if the connection is created without csfle options.