changes in internal usage of _root_ field cause failures to deleteById or duplicate docs when updating after upgrading from Solr7 to Solr8

We upgrade Solr7.7.2 to Solr8.8.2, keep using the Solr 7 index data, the query operation is fine. But when we try to add the doc (with the same doc id, actually is an update operation) to Solr8, we actually see 2 doc with the same id, which means the update did not remove the Solr7 doc.

Below is the schema.xml configuration for Solr7.

<field name="id" type="string" indexed="true" stored="true" required="true" multiValued="false" />
<uniqueKey>id</uniqueKey>
<field name="_root_" type="string" indexed="true" stored="false"/>

Add some fields in Solr8

<field name="nest_path" type="nest_path" stored="true"/>

<fieldType name="nest_path" class="solr.NestPathField" />

I can see in Sol7 code

DirectUpdateHandler2.updateDocOrDocValues

use the idTerm as updateTerm, but in this case Solr8 use rootTerm as the updateTerm. is this an expected behavior? how do we handle this incompatible issue? 

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This also impacts deletebyId