Node/Mongoose Error - Can't use $expr with String - node.js

Question

My goal is to create a query that searches for a partial string in a particular field - from the last index of a nested array in a MongoDB database. My constraint is that I must use a .find() query.
I'm using Mongoose 6.2.9 and MongoDB 5.0.4 - I've developed a query that works perfectly in MongoDB Compass, but Mongoose seems to be having issues with:
(node:48632) UnhandledPromiseRejectionWarning: Error: Can't use $expr with String.
The call I'm trying to make:
findQuery['statuses.schema'] = {
$expr: {
$regexMatch: {
input: {
$arrayElemAt: ['$statuses.schema', -1]
},
regex: "sample|regex|here",
options: 'i'
}
}
};
The issue seems to be placing $statuses.schema inside quotation marks (cast as a string) - but without these quotation marks, I seem to get an error in my GraphQL response (this query is part of a resolver for a GraphQL API) that would lead me further away from a solution:
{
"message": "$statuses is not defined",
"locations": [
{
"line": 2,
"column": 3
}
],
Edit: Added Schema
const statusSchema = mongoose.Schema({
timestamp: { type: String },
status: { type: String },
test: { type: String },
symptom: [symptomSchema],
code: { type: String },
});
const serverSchema = mongoose.Schema({
ipAddress: { type: String },
index: { type: String },
serialNumber: { type: String },
model: { type: String },
configCode: { type: String },
operator: { type: String },
onlineStatus: {type: Boolean, default: false},
repairCount: { type: String },
lastRepair: { type: String },
customer: { type: String },
location: [locationSchema],
inRepair: { type: String },
rack: { type: String },
statuses: [statusSchema],
schema_version: { type: String },
}, { _id: true});
So in short, MongoDB seems to require "$myArray.myField" be a string, but Mongoose cannot handle it being a string when nested under $expr. What am I doing wrong?