Salem

I was trying to build one, but after it have reached 100% progress, it have demanded another 10 dry boards, so I've added 2 and progress was over 100%. Now it demands from me more building materials. Is that good or not? Does it work correctly?

Definitely sounds bugged. If you can, leave it as it is so Loftar can have a look at it when he comes back (I think the devs are on a holidays break). PM him about it so he doesn't miss this thread.

Now the project is totally gone, great!

If the sign wasn't on a claim with paid upkeep, it can still decay. I seem to recall something similar to this being reported before. The sign takes a decay hit and loses some of the built materials. I'm not sure anything had been pointed out as being a problem at the time and there weren't any follow-ups about reproducing this.