I found out the cause of this. It is because WorldAnchors and Concrete share the same block ID (different metadata, worldanchors being 214:0 and Concrete being 214:1). There is a plugin which restricts the placement of worldanchors, to 1 per player. This seems to have also effected Concrete (same block ID issue). I've made some changes to this to see if it works, but it may require a server restart, which will likely happen when the server becomes empty at some point.
Thanks for letting us know.