SQLite Forum

Concurrency not working
"Writers merely append new content to the end of the WAL file. Because writers do nothing that would interfere with the actions of readers, writers and readers can run at the same time. However, since there is only one WAL file, there can only be one writer at a time."

From Section 2.2 of: https://sqlite.org/wal.html#activating_and_configuring_wal_mode