http://beta.musicbrainz.org/edit/19194010 was entered after your edit and appears to have made a change to the artist, although I can't tell what.

I'm pretty sure it just changed the recording title, not the artist (and therefore shouldn't conflict)

If it had only changed the recording title, the artist section wouldn't be shown. Compare it with http://beta.musicbrainz.org/edit/19245554 for example.

Join phrase – the initial edit changed a join phrase of "" to " ".

So, should this be closed as invalid?

I'm not sure. I'd be curious to know why it is that the join phrase was getting changed, and if it's still possible for it to change that way. I think we have a patch in the pipeline that would make this impossible (having whitespace in the final join phrase), at the database level. However, it still seems like the conflicting edit shouldn't be possible (rather than it just being impossible to apply the edit, like the patch I mention does). So I guess more investigation is needed

This is no longer possible due to the controlled_for_whitespace checks.