Some, but not all, of the quoins in Groddle Forest Junction go invisible for me after I leave the street (either /home or to an adjacent street) and come back. This has happened for two successive logins; I can see them when I first log in, and even after the invisibility I can still see other quoins of the same type, so it's not an issue of the image not loading.
If I run into an invisible quoin, I get the points. After running into the same invisible quoin several to many times, it may start to regenerate as visible again ... but not at the same rate. There are two (invisible) imagination quoins on the same branch and I never got one without the other, but one became re-visible before its neighbor did.