Doing FabricBlockSettings.copyOf(Blocks.HONEY_BLOCK) makes it so the block these settings are used on still has normal jump height on top of it. Adding .jumpVelocityMultiplier(0.5f) manually fixes it, however this implies that jumpVelocityMultiplier is not copied.

Keep in mind that the vanilla implementation does not copy jumpVelocityModifier either, alongside other fields such as the various context predicates, likely because no vanilla block requires them to be copied. I would change the Fabric API method to copy these fields, but also add a note that additional fields may be copied compared to vanilla's implementation.

That's probably an oversight. Walking speed carries over thus it follows that jump height should carry over.