Can't call AppEngine Image API getServingUrl method when using SpringBoot stack

Question

Our project needs to serve scaled images. AppEngine ImageApi has a wonderful method called getServingUrl. But this API is only accessible from AppEngine.

Problem

I've created a PoC project with SpringBoot and gradle for AppEngine. And whenever I call a simple endpoint that should call getServingUrl I get the following error:

Can’t make API call blobstore.CreateEncodedGoogleStorageKey in a thread that is neither the original request thread nor a thread created by ThreadManage

But when I do a similar setup that is based on Servlets, it works as expected.

SpringBoot Setup

Here is an example of some files from my setup. build.gradle:

;; https://github.com/GoogleCloudPlatform/app-gradle-plugin
buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath 'com.google.cloud.tools:appengine-gradle-plugin:2.2.0'
    }
}

plugins {
    id 'org.springframework.boot' version '2.2.6.RELEASE'
    id 'io.spring.dependency-management' version '1.0.9.RELEASE'
    id 'java'
}

group = 'com.example'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '11'

apply plugin: 'com.google.cloud.tools.appengine'  // App Engine tasks

repositories {
    mavenCentral()
}

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-web'
    implementation "com.google.appengine:appengine-api-1.0-sdk:2.0.4"

    testImplementation('org.springframework.boot:spring-boot-starter-test') {
        exclude group: 'org.junit.vintage', module: 'junit-vintage-engine'
    }
}

test {
    useJUnitPlatform()
}

appengine {  // App Engine tasks configuration
    deploy {   // deploy configuration
        projectId = 'GCLOUD_CONFIG' // delegate to project in gcloud config
        version = 'GCLOUD_CONFIG'   // delegate to gcloud to generate a version
    }
}

appengine/app.yml

runtime: java11

Controller

package com.example.demo.controller;

import com.google.appengine.api.images.ImagesServiceFactory;
import com.google.appengine.api.images.ServingUrlOptions;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class ImageController {

    @RequestMapping("/image-url")
    public ImageUrlDto hello(@RequestParam String bucket, @RequestParam String image) {
        ServingUrlOptions options = ServingUrlOptions.Builder
                .withGoogleStorageFileName(String.format("/gs/%s/%s", bucket, image));
        return new ImageUrlDto(ImagesServiceFactory.getImagesService().getServingUrl(options));
    }
}

class ImageUrlDto {
    String url;
    public ImageUrlDto(String url) {
        this.url = url;
    }
}

Question

Servlet setup is working correctly, but spring setup no. I've read that Image API is accessible only within AppEngine standart environemnt and request thread/ThreadManager spawned thread.

Am I missing something for that Spring setup? Probably that requires some another form of deployment or thread configuration in order to work with Image API?