This uses the fulltext search index, and MySQL has no stemming. T4818 discusses stemming in more detail.

We can swap to ElasticSearch to get stemming, but this has yet to annoy me enough to motivate me to set that up, figure out exactly how to configure it, and commit to maintaining it. If it's a big pain rather than a minor annoyance, yell at me. I imagine we'll set it up before SAAS happens.

We could possibly write our own stemmer and pre-filter text before shipping it to the MySQL indexer, but that's also a bunch of work.

Upshot: MySQL fulltext indexes aren't that great; installs can already resolve this through configuration; I'm not planning to resolve it for this install specifically because I'm not annoyed enough by it.