That's due to not changing the music call after the boss. Cinossu's 16-bit rendition of Bridge was being used for Green Hill before Tweaker and Puto's Jungle was inserted.
Green Hill plays Jungle Zone from Sonic 1 8-bit
After act 3 it plays Bridge Zone.
Yes, but not for bosses, it'll be used for something else.
Also is there any plans to implement the 8-bit boss music?