Deleting audio files on FR255

Hi all, I need to free some space on my FR255 as there are too many audio tracks on it and I have no more space to upload new audio tracks.

So I used Garmin Express to remove some tracks from the 255.

Doing this, I discovered that in Garmin Express many tracks on the watch are not well indicated (i.e. artist name & track title - see below picture) but just some short words that doesn't allow me to recognize what track is it (com]). So it was impossible for me to delete anything as it could be deleting others that I wanted to keep, instead. Look at the image:

The second track is effectively not recognizable. No additional informations except the track length. No scrolling informations. What could be the problem?

So I tryed to delete by PC, assuming the FR255 as an external memory, I went in the directory /Garmin/Music and finally I clearly found the tracks I wanted  to delete.

But, very strange, I deleted from PC...  but the tracks are still on the watch if I check on Garmin Express! 

Anyone has any idea where's the problem?

Thank you !