That's actually intended, a whole glow didn't look good, so I made it like "fade" on each side. There is room to improve the quality of the fade maybe, I'll look into it.
Funny that you didn't notice another thing that is actually wrong there, precisely because of that fade, the beginning of the middle section overlaps with the end of the first section (see screenshot). That probably happens with the end section too, but is not seen because it's covered by the seek thumb.