Reading the article, it seems that the problem is not that the missles disappeared - they were decommissioned. The question seems to be that the results of the intended recycling operations cannot be verified. Are the missle bodies just lying around, did they get recycled and the valuable materials sold on the black market, what? They current Ukrainian administration can’t say.
It’s worrying, but probably not worth a panic. Not much you can do with an ICBM without the launch infrastructure.