GKE phpMyAdmin shared sessions do not work with Filestore (NFS)

I am migrating phpMyAdmin off of an old Debian server and into Kubernetes using GKE. I have created a Docker image that runs phpMyAdmin with apache2. The web page comes up and I can log into various MySQL databases and make some queries, but after only a couple of minutes the queries stop working and I get the error message "Error in processing request / Error code:502 / Error text: Bad Gateway (rejected)".

The session storage writes to an attached GCP Filestore (NFS) volume. If I change the session storage to write to local directory rather than NFS things start working.

I would prefer to use the shared NFS storage as this allows me easily cluster multiple back-end instances of phpMyAdmin. Is there any way I can configure phpMyAdmin to support writing session information into NFS avoiding these problems?

Here is the phpMyAdmin configuration:

$cfg['AllowUserDropDatabase'] = FALSE;
$cfg['AllowArbitraryServer']  = FALSE;
$cfg['ShowCreateDb']          = FALSE;

$cfg['Servers'][1]['host']         = '127.0.0.1';
$cfg['Servers'][1]['verbose']      = 'mysql-srv1.example.com';
$cfg['Servers'][1]['port']         = '3306';
$cfg['Servers'][1]['auth_type']    = 'cookie';
$cfg['Servers'][1]['pmadb']        = '';
$cfg['Servers'][1]['DisableIS']    = true;

$cfg['Servers'][2]['host']         = '127.0.0.1';
$cfg['Servers'][2]['verbose']      = 'mysql-srv2.example.com';
$cfg['Servers'][2]['port']         = '3316';
$cfg['Servers'][2]['auth_type']    = 'cookie';
$cfg['Servers'][2]['pmadb']        = '';
$cfg['Servers'][2]['DisableIS']    = true;

$cfg['UploadDir'] = '';
$cfg['SaveDir'] = '';

$cfg['blowfish_secret'] = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx';
$cfg['SessionSavePath'] = '/var/tmp/phpmyadmin';