optical flow not rendering with 'render in and out'

When I slow down a clip and set it to optical flow, make my in and out selection in timeline and select 'render in and out' it renders but optical flow doesn't take effect. The footage is still choppy. I know optical flow is applied to the clip because if I export the in and out sequence, it very clearly has optical flow on it. I have a core i9 13900K, 4090, and 32GB RAM so hardware is clearly not the issue here.