Nope, I have no idea, sorry!
I do dimly remember the thing about the files only appearing when you select 'all', as if rockbox no longer recognises them as playable file formats, happening once years ago, but can't remember the context or reason for it. And the "._ "prefix seems like the sort of thing Macs do when they want to hide a file. The files just suddenly acquired that prefix for no obvious reason?
I assume you've tried copying all the music across again? Also maybe reinstalling the rockbox folder manually? Kind of still feels like a corruption issue.