Rendered mix is not the same as mix inside Reaper--track folders?

Today I have been struggling to render a correct mix of a song I'm working on, and it's become apparent that Reaper treats track folders in some unexpected way for the purposes of rendering (or there is some setting I don't know I have enabled that is causing this to happen). I have a mix with two guitar tracks in a folder, eight drum tracks in a folder, several synth tracks in a folder, and finally bass and drum tracks outside folders. No matter what I do, when I render a stereo mixdown, the mix is completely different than what I hear in Reaper--the bass and vocals are both way louder than they should be, and the guitar and synth are way quieter. I tried putting the bass and vocal tracks each in their own folders so that everything in the final mixdown originated from inside a folder, but it didn't change anything. The only way I could get the rendered output to match what I hear while playing the mix inside Reaper was to eliminate all use of track folders. What is going on here?

Does the problem still exist if you bounce instead of render?

You could send us just the project file (upload to this thread) and we can check your routing for you.