php7.4 unable to read memcached data saved by pylibmc

I keep getting these errors trying to read memcached variables stored by python3 scripts runnnig pylibmc.

PHP Warning: Memcached::get(): could not decompress value: unrecognised compression type in xxx

I’m saving some test data using this python3 script:

#!/usr/bin/env python3
import json
import pylibmc

mc = pylibmc.Client(["127.0.0.1"], binary=True, 
     behaviors={"cas": True, "tcp_nodelay": True,"ketama": True})

mc.set('testvar', json.dumps('{"greeting": "Hello", "title": "Mr"}'), 3600)

Using telnet to fetch the variable from memcached shows the data is NOT compressed, but saved in clear text:

ubuntu@server:~/bin$ telnet localhost 11211
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
get testvar
VALUE testvar 16 81
"{"greeting": "Hello", "title": "Mr"}"
END
^]
telnet> quit
Connection closed.

I have set the compression limit in the php-memcached settings to a huge number:

/etc/php/7.4/cli/conf.d/25-memcached.ini contains:

extension=memcached.so
; You need to install php-igbinary package to use igbinary serializer
; and php-msgpack to use msgpack serializer
memcached.serializer=php
memcached.compression_threshold=9999999999

The PHP script trying to read the data:

<?php
    echo ini_get("memcached.compression_threshold") . "n";
    $mc = new Memcached();
    $mc->addServer('localhost', 11211);

    $data = json_decode($mc->get('testvar'));
    echo $data;
    echo "n";

?>

and the output:

9999999999
PHP Warning:  Memcached::get(): could not decompress value: unrecognised compression type in /home/ubuntu/bin/memcache_get.php on line 6

This is on a freshly installed Ubuntu 20 LTS system with python 3.8.10 and pylibmc Version: 1.6.1

Any hints anyone?