Spring Boot app deployed in Google Cloud App Engine not starting up because of memory limit

I'm having a problem with a Spring Boot application deployed in Google Cloud App Engine. The app is an API that uses JPA and JWT and is connected to a MySQL database stored in Google Cloud SQL.

The problem is that the applicacion gets stuck because of memory limit. After every request, I get this messages in the log:

• Exceeded soft memory limit of 256 MB with 298 MB after servicing 0 requests total. Consider setting a larger instance class in app.yaml.
• This request caused a new process to be started for your application, and thus caused your application code to be loaded for the first time. This request may thus take longer and use more CPU than a typical request for your application.
• While handling this request, the process that handled this request was found to be using too much memory and was terminated. This is likely to cause a new process to be used for the next request to your application. If you see this message frequently, you may have a memory leak in your application or may be using an instance with insufficient memory. Consider setting a larger instance class in app.yaml.

I tried to modify the file src/main/appengine/app.yaml file in order to set a different configuration (with more memory) but I don't see any difference after every change. It's like this file was ignored.

This is my current app.yaml:

runtime: java
env: flex
runtime_config:
  jdk: openjdk8
env_variables:
  SPRING_PROFILES_ACTIVE: "gcp,mysql"
  # JAVA_GC_OPTS: -XX:+UseSerialGC
  # JAVA_USER_OPTS: -XX:MaxRAM=200m

# With -XX:+UseSerialGC This will perform garbage collection inline with the thread allocating the heap memory instead of a dedicated GC thread(s)
# With -Xss512k This will limit each threads stack memory to 512KB instead of the default 1MB
# With -XX:MaxRAM=72m

handlers:
- url: /.*
  script: this field is required, but ignored
beta_settings:
  cloud_sql_instances: guitar-tab-manager-api:europe-west3:guitar-tab-manager-db

# manual_scaling:
#   instances: 1

# instance_class: F4
# manual_scaling:
#   instances: 1

# instance_class: F2
# basic_scaling:
#   max_instances: 5
#   idle_timeout: 10m

instance_class: F2
# automatic_scaling:
#   target_cpu_utilization: 0.65
#   min_instances: 5
#   max_instances: 100
#   min_pending_latency: 30ms  # default value
#   max_pending_latency: automatic
#   max_concurrent_requests: 50

I tried to apply different configurations but nothing seems to work. Maybe someone can help. Thanks in advance.

For anyone coming here:

I got it working using a different environment. Instead of flex, I changed the environment to standard with java8 following the configuration explained here. The weird thing is that now the app consumes more memory than before (about 300Mb), but now it's working without issues. Note that app.yaml is not used anymore, and now the application is deployed as a WAR.

Thanks