I know that Beetle psx and bette saturn are fork from mednafen. Today the last version of mednafen is 1.26.1 Are the core in mednafen up to date compared to this version or the branch in libretro project have his own life since it was forked?
Thank for any explanation 
Beetle Saturn is not. I think Beetle PSX might be
Not sure whether you’re still looking for an answer for this, but it appears that Beetle PSX is not up to date. For example, it doesn’t seem to include the SPU reverb fixes from Mednafen 1.26.1, which significantly improve the sound in a lot of games (most notably Final Fantasy VII).