@Facefister Did you check damage, or just character sheet? There are a number of passives that fail to find their way to the character sheet, the damage is added however.
Can confirm - things are working fine with Silver Bolts, but as soon as you morph it to Silver Leash, the Slayer bonus goes away. Both tooltip and character sheet.
(also happens if you don't put points into Slayer until after you have Silver Leash)
Happening on PS4. Just checked my character and only have 3% bonus with Slayer maxed and both Dawnbreaker and Silver Leash equipped. Goes up to 6% if I switch Leash with Trap.