S3 presigned post urls issue using boto3

I have a webapp served with apache2 running python-flask in the backend. The app heavily relies on the S3 Object Storage. I'm using boto3 to interact with the S3 storage. My issue is regarding the generate_presigned_url method when used in production. It returns the following structure:

{
 'url': 'https://eu-central-1.linodeobjects.com/my-s3-bucket', 
 'fields': {
   'ACL': 'private', 
   'key': 'foo.bar', 
   'AWSAccessKeyId': 'FOOBAR', 
   'policy': 'base64longhash...', 
   'signature': 'foobar'
  }
} 

Everytime I use this method on the same python session the policy key returns a longer value (about 1.5x increase in length for every subsequent request). After a few requests the size of the policy gets really large (tens of MB) and the app breaks. If I restart the python service the policy size gets reset.

After digging in the boto3 documentation and some threads in GitHub and here I couldn't find anything that helped me in regards to resetting the S3 connection without having to restart the whole python session. To keep restarting the apache2 service periodically is not a good approach, so my solution was to call the generate_presigned_url from a standalone script using subprocess and parse the string output back to json before using it, which is not ideal, as I wish I didn't have to keep calling bash scripts from inside apache. The main functions I use follow bellow:

AWS_BUCKET_PARAMS = {'ACL': 'private'}

# connect to my linode's s3 bucket
def awsSign():
    return boto3.client('s3', aws_access_key_id=AWS_ACCESS_KEY_ID, aws_secret_access_key=AWS_SECRET_ACCESS_KEY, endpoint_url=AWS_ENDPOINT_URL)

# generate presigned post object for uploading files
def awsPostForm(file_path):
    s3 = awsSign()
    return s3.generate_presigned_post(AWS_BUCKET, file_path, AWS_BUCKET_PARAMS, [AWS_BUCKET_PARAMS], 1800)

# generate post object from external script
def awsPostFormTerminal(file_path):
    from subprocess import Popen, PIPE
    cmd = [ 'python3', '-c', f'from utils import awsPostForm; print(awsPostForm("{file_path}"))' ]
    output = Popen( cmd, stdout=PIPE ).communicate()[0]
    return json.loads(output.decode('utf-8').replace('\n', '').replace("'", '"'))

The problem happens regardless of calling awsSign() one or many times for a list of files.

In short, I wish for a better way of retrieving subsequent post forms from generate_presigned_url in the same python session, without increasing the policy on every new request. If there is a proper way to restart the boto3 connection, provide some parameters that I missed when setting the API calls or maybe it's something particular to the Linode's S3 object storage service.

I also posted the same question on stackoverflow.

If anyone can point me at the right direction I'll appreciate!