xsi:type="foreign" not working in db_schema.xml in magento 2

Question

This is my db_schema.xml file

<table name="plenty_item_import_item" comment="Plenty Item Import Item Table" engine="innodb" resource="default">
    <column name="entity_id"       xsi:type="int" identity="true" padding="10" unsigned="true" nullable="false" comment="Entity Id" />
    <column name="item_id"         xsi:type="int" padding="10" unsigned="true" comment="Item Id" />
    <column name="variation_id"    xsi:type="int" padding="10" unsigned="true" comment="Variation Id" />
    <constraint xsi:type="primary" referenceId="PRIMARY" >
        <column name="entity_id"/>
    </constraint>
</table>

<table name="plenty_item_import_item_bundle" engine="innodb" resource="default" comment="Plenty Item Import Item Bundle Table">
    <column name="entity_id"           xsi:type="int" padding="10" unsigned="true" nullable="false" identity="true" comment="Entity Id" />
    <column name="variation_bundle_id" xsi:type="int" padding="10" unsigned="true" nullable="true" comment="Variation Bundle Id" />
    <column name="item_id"             xsi:type="int" padding="10" unsigned="true" comment="Item Id" />
    <constraint xsi:type="primary" referenceId="PRIMARY" >
        <column name="entity_id"/>
    </constraint>
    <constraint xsi:type="foreign" referenceId="PLE_IMP_BUN_ITM_PLE_IMP_ITM" 
                table="plenty_item_Import_item_bundle" column="item_id" 
        referenceTable="plenty_item_Import_item" referenceColumn="item_id" onDelete="CASCADE"/>

</table>

In second table I am trying to add a foreign key, in this line

<constraint xsi:type="foreign" referenceId="PLE_IMP_BUN_ITM_PLE_IMP_ITM" 
            table="plenty_item_Import_item_bundle" column="item_id" 
    referenceTable="plenty_item_Import_item" referenceColumn="item_id" onDelete="CASCADE"/>

Now when I run the command

php bin/magento setup:db-declaration:generate-whitelist --module-name=Plenty_Item

to generate db_schema_whitelist.json file, then it was not generating. I checked online for this and I found if there is an issue in db_schema.xml file then json will not generate and I can also debug db_schema.xml file for errors. I got the answer from this link Magento 2 is not generating the db_schema_whitelist.json file via CLI

Now when I checked the errors then it was showing

2019-09-18T15:02:57+00:00 INFO (6): Notice: Undefined index: plenty_item_Import_item in D:\xampp\htdocs\magento-2.2\vendor\magento\framework\Setup\Declaration\Schema\Declaration\SchemaBuilder.php on line 352

It is showing undefined index for plenty_item_Import_item. Now when I remove the line to add the foreign key then it works perfectly. Now my question is How and why it is showing undefined index for plenty_item_Import_item as this table is added at top. I also checked db_scema.xml for catalog module that is present by default and found they have also added a foreign key by same method. Code from db_schema.xml present in module-catalog

<constraint xsi:type="foreign" referenceId="CAT_PRD_ENTT_DTIME_ENTT_ID_CAT_PRD_ENTT_ENTT_ID"
                table="catalog_product_entity_datetime" column="entity_id" referenceTable="catalog_product_entity"
                referenceColumn="entity_id" onDelete="CASCADE"/>

Answer 1

I would guess it is because of your typos in the constraint. Both table names have a capital I for _Import_ instead of a small letter _import_. Nothing else seems to be wrong.