The bug
Double slabs have the waterlogged tag. In return, double slabs have waterlogged versions of them in debug mode.
MC-128390
Double Slabs have waterlogged variants in Debug Mode
Resolved
Works As Intended
0
Play Dash Number f800f8 000000
(Unassigned)
Unconfirmed
double-slabs, waterlogged-blocks
Minecraft 18w15a
Retrieved
Slabs have 2 block states: type=top|bottom|double and waterlogged=true|false.
The debug world shows every single combination; even impossible ones (like waterlogged double slabs, or closed, powered iron (trap)door); that is by design.