Adding new Deserializer to Rest API removes other Deserializers

[jakwinkler]

Preconditions (*)

1. Magento 2.2.x / Magento 2.3.x both CE / EE & 2.4-develop

Steps to reproduce (*)

1. Create a new module which will add new webapi route
2. Create a new deserializer using di.xml in your module

<type name="Magento\Framework\Webapi\Rest\Request\DeserializerFactory">
    <arguments>
        <argument name="deserializers" xsi:type="array">
            <item name="text_plain" xsi:type="array">
                <item name="type" xsi:type="string">text/plain</item>
                <item name="model" xsi:type="string">Custom\Module\Webapi\Rest\Request\Deserializer\Plain</item>
            </item>
        </argument>
    </arguments>
</type>

Expected result (*)

1. New content type accepted by REST API

Actual result (*)

1. All CORE defined content types for API are no longer working and return 400 error
   

2. Only new defined content type is accepted

It was my understanding that DI.XML is used to extend CORE features, not OVERWRITE them.

[m2-assistant]

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[jakwinkler]

After checking content of
generated/metadata/global.php
DeserializerFactory has only 1 argument, defined in my module, others are missing.

'Magento\\Framework\\Webapi\\Rest\\Request\\DeserializerFactory' => array ( 'objectManager' => array ( '_i_' => 'Magento\\Framework\\ObjectManagerInterface', ), 'deserializers' => array ( '_v_' => array ( 'text_plain' => array ( 'type' => 'text/plain', 'model' => 'Custom\\Module\\Webapi\\Rest\\Request\\Deserializer\\Plain', ), ), ), ),

[jakwinkler]

Its the same with etc/webapi_rest/di.xml

[m2-assistant]

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[jakwinkler]

Just an update from my end:
It seems like this is a general di.xml bug / (feature??)
If you try to add a new argument to an existing <type> in your own module, it will overwrite all arguments defined in CORE or 3rd party modules.
Its the same for all areas where you can define di.xml file
In order for this to work, you have to copy+paste arguments section to your own module

[engcom-Hotel]

Hello @qsolutions-pl
This issue is a bug.

Thanks for collaboration!