There's also issues with the state disagreement / resolution algorithms across federation.
Has this been solved? Maybe it's also due to database corruption, where some state is forgotten across the federation, and thus the algorithm breaks down?
It can baloon as it scales up. Matrix.org (homeserver) has had at least one DB corruption and that's with their proprietary Rust bindings for Synapse. Small communities, especially ones that share rooms between them, should be fine on most systems. Make regular backups of the DB.
67% made a match. 90% of matches were right.
No idea how they got that number, though.