[BUG] Generator 3.3.4 does not respect Java import mapping for string refs

Description

I am trying to map a custom data type to a JSON string-valued property (more or less equivalent to a custom format); Jackson will transparently handle the conversion for me in Java. However, the generator does not observe user custom mappings when a $ref type is marked as a string.

openapi-generator version

This behavior is observed with 3.3.4

OpenAPI content

# correctly identifies that EntityIdentifier is a string and produces a Map<String, String>,
# but does not use the LSP to use a custom data type
components:
  schemas:
    EntityIdentifier:
      type: string
      pattern: '^\w+:[\w\d-]+$'
    ResolveExpressionRequest:
      type: object
      properties:
        expression:
          type: string
          minLength: 1
        rootObject:
          type: object
          additionalProperties:
            $ref: '#/components/schemas/EntityIdentifier'

# Generates a Map<String, EntityIdentifierDto>
components:
  schemas:
    EntityIdentifier:
      type: object
    ResolveExpressionRequest:
      type: object
      properties:
        expression:
          type: string
          minLength: 1
        rootObject:
          type: object
          additionalProperties:
            $ref: '#/components/schemas/EntityIdentifier'

Command line used for generation

Invoked via Maven plugin:

<plugin>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-generator-maven-plugin</artifactId>
    <version>3.3.4</version>
    <executions>
        <execution>
            <goals>
                <goal>generate</goal>
            </goals>
            <configuration>
                <inputSpec>${project.basedir}/src/main/openapi/openapi.yaml</inputSpec>
                <generatorName>java</generatorName>
                <importMappings>EntityIdentifier=foo.EntityIdentifierDto</importMappings>
                <configOptions>
                  <library>resttemplate</library>
                  <java8>true</java8>
                </configOptions>
            </configuration>
        </execution>
    </executions>
  </plugin>

Steps to reproduce

As above, a $ref that is of type: string does not get overridden by the custom mapping.

Use the Maven plugin configuration and supplied OpenAPI snippets to reproduce how the Java generator handles importMappings for a string-valued $ref in additionalProperties. Trace the Java generator's type-mapping path and add or run a regression test; done means the custom EntityIdentifier mapping is used for the generated Map value without changing ordinary string handling.