Weird Groups + Lemmy + Mastodon behaviour

If I post a message to @spurs and @coys (specifically in that order) on Mastodon, the post somehow ends up here:

lemmy.world/c/spurs@chirp.social

(there’s no mod for a start)

This is where I am expecting the post to end up:

lemmy.world/c/coys

This is a bit confusing and undesirable. Looks like Lemmy creates a board for non-Lemmy fedi groups and uses the first group mentioned as the destination for the post, rather than any subsequent groups mentioned, even if they are an existing Lemmy board…?

If I mention the groups in the reverse order (@coys first and @spurs second) then the Masto post ends up on the Lemmy board I’d expect.

It’s extremely weird and confusing. Anyone got any insights/thoughts on this behaviour?