useQuery's onCompleted being called with cached value - reactjs

Question

Hopefully I can articulate this question clearly without too much code as it's difficult to extract the pieces from my codebase.
I was observing odd behavior yesterday with useQuery that I can't seem to understand. I think I understand Apollo's cache pretty well but this particular behavior doesn't make sense to me. I have a query that looks something like this:
query {
reservations {
priceBreakdown {
sections {
id
name
total
}
}
}
}
The schema is something like:
type Query {
reservations: [Reservation]
}
type Reservation {
priceBreakdown: PriceBreakdown
}
type PriceBreakdown {
sections: [Section]
}
type Section {
id: String
name: String
total: Float
}
That id on Section is not a proper ID and, in fact, is not unique. It's just a string and all PriceBreakdowns have a list of Sections that contain the same ID. I've pointed this out to the backend folks and it's being fixed but I realize this causes incorrect caching with Apollo since there will be collisions w.r.t. __typename and id. My confusion comes from how onCompleted is called. I noticed when doing
const { data } = useQuery(myQuery, {
onCompleted: console.log
})
that when the network call returns, all PriceBreakdowns are unique and correct, as they should be. But when onCompleted is called with what I thought would be that same API data, it's different and seems to reflect the cached values. In case that's confusing, here are the two results. First is straight from the API and second is the log from onCompleted:
// api results
"data": [
{
"id": "92267",
"price_breakdown": {
"sections": [
{
"name": "Reservation",
"total": "$60.00",
"id": "RESERVATION"
},
{
"name": "Promotions and Fees",
"total": null,
"id": "PROMOTIONS_AND_FEES"
},
{
"name": "Total",
"total": "$51.00",
"id": "HOST_TOTAL"
}
]
}
},
{
"id": "92266",
"price_breakdown": {
"sections": [
{
"name": "Reservation",
"total": "$30.00",
"id": "RESERVATION"
},
{
"name": "Promotions and Fees",
"total": null,
"id": "PROMOTIONS_AND_FEES"
},
{
"name": "Total",
"total": "$25.50",
"id": "HOST_TOTAL"
}
]
}
}
]
// onCompleted log
"data": [
{
"id": "92267",
"price_breakdown": {
"sections": [
{
"name": "Reservation",
"total": "$60.00",
"id": "RESERVATION"
},
{
"name": "Promotions and Fees",
"total": null,
"id": "PROMOTIONS_AND_FEES"
},
{
"name": "Total",
"total": "$51.00",
"id": "HOST_TOTAL"
}
]
}
},
{
"id": "92266",
"price_breakdown": {
"sections": [
{
"name": "Reservation",
"total": "$60.00",
"id": "RESERVATION"
},
{
"name": "Promotions and Fees",
"total": null,
"id": "PROMOTIONS_AND_FEES"
},
{
"name": "Total",
"total": "$51.00",
"id": "HOST_TOTAL"
}
]
}
}
]
As you can see, in the onCompleted log, the Sections that had the same ID as Sections from the previous record are duplicated, suggesting Apollo is rebuilding the payload from cache and calling onCompleted with that. Is that what's happening? If I set the fetchPolicy to no-cache, the results are correct, but of course that's just a patch for the problem. I want to better understand Apollo because I thought I understood and now I see something unintuitive. I wouldn't have expected onCompleted to be called with something built from the cache. Thanks in advance.