I've noticed this new issue as well. Strangely, an old blueprint stored in a projector from more than 2 weeks ago (my repair projection) would work just fine, but when I added a new projector to start building an OSPREY (https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=936622135&searchtext=osprey) it refused to place any projected blocks. I had to place the block, and then the Nanites would construct it. Later after getting it mostly finished, I separated the ships, added a projection beacon to the new ship, aligned a new projector, and Nanites still would not construct the projected blocks, but WOULD repair manually placed blocks.

I also tested and found that the NBRS mod (https://steamhost.cn/steamcommunity_com/workshop/filedetails/?id=857053359) similarly failed to place projected blocks, but would weld existing blocks. Maybe there's a recent bug Keen added to break this? If so, I can't find it on the Keen forums.