Hi Michael, in Resolve every grade is applied based on its source clip and we use timecode and other items like Reel name or File name to separate similar timecodes. In your case there is one clip so it gets one grade. This is ideal for many situations but clearly when you have one long clip you need to apply different grades. The way to do this is with grade versions.
You can split the clips in a number of ways to make separate clips, scene cut or manual split or XML, but ultimately by making a ‘version’ of the grade for each section of the clip will permit multiple grades for the one source clip. Leave the wanted version for each section active and render all the clips. The render should create one clip per section each with the correct version.
Peter