Thanks for finding this!
What do you think about such change:

if (
    ($distributedCacheClass === '' && $localCacheClass !== \OC\Memcache\APCu::class)
    || ($distributedCacheClass !== '' && $distributedCacheClass !== \OC\Memcache\APCu::class)
) {
    $this->cache = $cacheFactory->createDistributed(Application::APP_ID . '/service');
}

For the case when memcache.distributed=`` and memcache.local=`APCu` ?

What do you think about such change

Yes, makes sense. Otherwise we would end up again with APCu as the cache. I updated the PR accordingly.

Now I am wondering, would we need to check $localCacheClass !== '' as well, or is it fine for your use-case?

Edit: Seems to be fine from a quick glance. In that case the NullCache is assigned, which would return null on get anyway and we fallback to querying the db.

/backport to stable32

/backport to stable31

@SystemKeeper can you please initiate backports? looks like bot ignores me 🤔

It's not you, the bot is down :-(

/backport to stable32

/backport to stable31