Not working auto remove failed server

[kansey]

We have two independent Yii2 applications that cache the database schema using Memcached.

Each application is configured with two Memcached servers:

App 1: memcached-server-1, memcached-server-2

App 2: memcached-server-2, memcached-server-1 (reverse order)

Problem:

If memcached-server-1 goes down → everything works fine, caching continues on memcached-server-2.

If memcached-server-2 goes down → caching stops working for both applications.

I expected that when one Memcached server is down, the surviving server should continue to store and retrieve the cached DB schema.

Configuration:

Memcached::OPT_REMOVE_FAILED_SERVERS = true is enabled in both applications.

Using consistent hashing (Ketama compatible).

Question:

Why does caching stop working when memcached-server-2 is down?

Why don't the applications re-cache the missing data to the remaining available server (memcached-server-1)?

Is this behavior expected? Could this be a bug in the php-memcached extension or libmemcached?

php memcached 3.1.5
libmemcached 1.0.18