If a holder contains a block that's supposed to be replaced with a transformer (such as Solid 0, Solid 1 or Space 0), it will just place that block and no transformation will occur.
The same thing happens to script blocks, they are just being placed, but not executed.

While this isn't strictly a bug, it definitely decreases possibilities of the holders.

Yep, you're right. I'll be looking into a solution for this.

Fixed in 452516f.