The bug
When creating a block tag in a datapack, attempting to extend a built-in block tag list with the minecraft prefix throws an error. However, leaving out the minecraft prefix works fine.
For example:
#minecraft:slabsDoes not work, but:
#slabsdoes work.
Indeed, I have the same problem in 18w21b, but the "#slabs" doesn't work for me.
An example:
[media]