Hydrocracker bottoms is a very crackable material, generally paraffinic. If you’re seeing coking in the reactor there’s a good chance that cracked products are condensing on the walls of the “cooler” reactor. If the feedstock does not fully vaporize you can condense this material in the reactor especially if the riser residence time is short and reactor residence time is long.
Seeing coking/fouling in the slurry circuit when the hydrocracker bottoms is being processed also points to uncracked feedstock making it’s way out of the reactor, staying with the slurry product. As the slurry inventory “cooks” at high temperature, the uncracked hydrocracker bottoms will thermally crack/degrade which leads to the fouling phenomena. Refiners that have changed their feed distributor technology to a more advanced design have seen this source of fouling go away when operating with the new technology. Highly crackable, paraffinic feedstock from crude will react similarly to the hydrocracker bottoms.