It's happening for me as well - upgraded a project from 16.2 to 17b8 and cropping, cropping softness and composite mode seems bugging for me on the render. Composite works in stills but not during preview/rendering playback.
I am essentially cropping portions out of videos and reposition them/overlaying them onto each other - sometimes 3-4 cropped overlays in parallel.
With 16.2 it just worked without blending issues but now in a small example it looks like this:

- Screenshot 2021-02-07 at 11.53.00.jpg (192.33 KiB) Viewed 4203 times
Clip is overlayed onto clip in the example picture above. Apparently the softness option in the cropping also is bugged as it starts to add light to the softened portion and on the edges it shows a border/sharp line.