It's possible to have a hidden volume envelope active. If you create one (or one gets auto created by dragging a volume envelope having item across a track) and untick the show and arm boxes, it's still active.
Otherwise Reaper is 1:1 with render sources. I don't trust anyone or any software either, so I tested it myself way back! Null a render against the source to prove it to yourself. Mono renders follow the render settings as well. (Obviously rendering a stereo source to mono would be a submix to mono on the fly there.)
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