According to the wiki, conduits are supposed to attack hostile underwater mobs, as well as giving the player a buff.

The buff is working, but I've placed a conduit at the mouth of an ocean monument and the guardians are swimming all over it unharmed.

The full conduit structure is built, the conduit is activated. Hence, the effect still working.

Update: Also ignores drowned zombies

Update: Please mark this as resolved, it wasn't a bug. I had the structure built wrong so the conduit was active but wasn't killing mobs, required more blocks. Apologies