I posted here a few weeks ago about a fence I had stained that I wasn’t completely satisfied with and received great feedback to just leave it alone and not continue to fuss with it. Now I have a situation where part of the stain seems to be coming off from where the planks join together. My question is is this something to be expected

and is there anything I can instruct the painter to do to prevent this going forward? I am thinking hand staining those areas should do it, but not sure. I believe he stained the fence by spraying it. Thanks. Pics attached.