This is so called “parasitic curing of residual resin” effect- see this post (maybe you need to scroll down little bit when open the link)
In short - if the area where the supports are touching the object is close to horizontal, the resin can’t flow away easily and stays for longer in this place and becomes semi cured from the reflected laser light around. See the images in the link I sent you - it’s visible how the supports leave “shadows” on the object surface.
The solution is - tilt the part to avoid horizontal sections with supports in order to allow the resin to flow away, also if possible decrease the number of supports in horizontal area as each support “attracts” the rubber due to the surface tension effects.
The effect is not officially mentioned in FormLabs documents and the software is not taking it into account, it’s up to users how to deal with it.