Hi there!
There is an issue in the editor, that allows to use “Join” inside “Filter”. It should not be possible, editor should show an error. That is why your formula is not working. Will fix it with the next release.
About “Map” function. No luck with this. We do not want to complicate formula implementation right now. Just technical stuff. Formulas use fibery api, and it does not support such expressions.
At the moment the best solution is to use intermediate fields, just as you did. I think we will add a possibility to hide fields on UI soon, so it will not blow your UI.