Additional buckets getting inserted with sum & rate/increase for histograms

We're experiencing a weird issue with histogram buckets on a few of our GCP projects. The following query returns a different set of buckets on different projects, even though the config is the same:

```
sum(increase(nginx_ingress_controller_request_duration_seconds_bucket{exported_namespace=~"(production|staging)-receiving"}[5m])) by (le)
```

Running the above query on GCP project A returns the following buckets (note the added buckets):

Running it on GCP projects B + C returns the following buckets (note the additional buckets that get inserted):

This is only happening for a combination of `sum(increase(bucket)) by (le)` and `sum(rate(bucket)) by (le)`, but not for `sum(irate(bucket)) by (le)`.

Any idea what might be going on here?