The Solr queue is sometimes reported as empty even though the indexer is running

Steps to reproduce:

1. Go to Administration -> Search
2. Change the action to "Reindex", leave the selection on "entire farm".
3. Click "Apply", and immediately afterwards, click again on "Search" in the administration menu to reload the page

Expected result:

The queue size is non-zero.

Actual result:

The queue size is 0, before refreshing and showing a high number/the actual number of items to re-index.