Problem with atomic updates after upgrading from 8.11 to 9.6

[Jeremy Buckley - IQS-C]

After updating to 9.6.1, the following update is failing:

[{
  "id":"contracts 36F79718D0274 | 65 II C",
  "price_list_url" : { "set" : "https://prices.anywhere.com"; }
}]

Solr responds with:

{
    "responseHeader": {
        "rf": 1,
        "status": 400,
        "QTime": 4
    },
    "error": {
        "metadata": [
            "error-class",
            "org.apache.solr.common.SolrException",
            "root-error-class",
            "org.apache.solr.common.SolrException"
        ],
        "msg": "Unable to index docs with children: the schema must include
definitions for both a uniqueKey field and the '_root_' field, using the
exact same fieldType",
        "code": 400
    }
}

We do not have nested child documents, at least not intentionally. Schema
has:

<field name="id" type="string" indexed="true" multiValued="false"
omitNorms="true" omitPositions="true" omitTermFreqAndPositions="true"
stored="true" termVectors="false"/>
...
<field name="price_list_url" type="string" indexed="true" stored="true"
multiValued="false" />
...
<uniqueKey>id</uniqueKey>

There is no _root_ field defined in the schema, and it is
using ClassicIndexSchemaFactory.
We are running Solr Cloud, this collection has one shard and two replicas.

Any ideas what could be causing this error or how to fix it?

Thanks in advance!