(function() {
const t = db.find_and_modify;
t.drop();

assert.commandWorked(t.insert({x: 1, y: 1, z: 1}));

let result = t.findAndModify({
    query: {x: 1},
    'new': false,
    update: {$set: {randomField: true}},
    fields: {x: 1, y: 1}
});
assert.eq(result.z, undefined);
})(); 

This test will succeed when run in the 'core' suite. When run in retryable_writes_jscore_passthrough which retries each write at least once, it will fail. This is because this code only fetches the preimage/postimage of the document from the oplog, but does not re-apply the 'fields' projection. This means that the entire pre-image/post-image will always be returned.