Mongoose can find related documents with lookup but not populate - node.js

Question

I have a simple Mongo DB with two models: Campaign and Donation, where a Campaign is related to many Donations.
Using mongoose I can query for a specific Campaign using an aggregation with a lookup() and it returns all of the related Donations. But if I try to use a populate() statement, no Donations are returned. I cannot figure out why.
Campaign
const mongoose = require('mongoose');
const Schema = mongoose.Schema;
require('models/Donation');
const CampaignSchema = new Schema({
name: {
type: String,
required: true
},
goal: {
type: Number,
required: true
},
donations: [{
type: Schema.Types.ObjectId,
ref: "Donation"
}]
},
{
collection: 'campaigns'
});
mongoose.model('Campaign', CampaignSchema);
Donation
const mongoose = require('mongoose');
const Schema = mongoose.Schema;
require('models/Campaign');
const DonationSchema = new Schema({
campaign: {
type: Schema.Types.ObjectId,
ref: "Campaign",
required: true
},
amount: {
type: Number,
required: true
},
date: {
type: Date,
required: true,
default: Date.now
}
},
{
collection: 'donations'
});
mongoose.model('Donation', DonationSchema);
This code will return a single Campaign based on _id and all of the associated donations:
const filter = {_id: mongoose.Types.ObjectId(req.params.id)};
Campaign.aggregate().match(filter).lookup({
from: "donations",
localField: "_id",
foreignField: "campaign",
as: "donations"}).then((results) => {
console.log(results[0].donations); // HAS ALL OF THE RELATED RECORDS
});
This code returns an empty array for donations:
const filter = {_id: mongoose.Types.ObjectId(req.params.id)};
const campaign = await Campaign.findOne(filter).populate('donations').then((c) =>{
console.log(c.donations); // EMPTY
});