How does the search function work and why does it sometimes not work from different instances when searching All?

In other words -

• I search for a community that exists on Lemmy instance A (e.g. midwest.social) from Lemmy instance B (e.g. lemmy.ml) and it finds it.
• I search for that same community from Lemmy instance C (e.g. lemmy.one), and it doesn’t find it.
• Lemmy instance A, B, and C are all connected / federated.