Sculk Catalyst limited blooming when death is on top of Catalyst
Expected behavior: Sculk blooming to blocks under and around (at level) the sculk catalyst when mobs die on top of sculk catalyst.
Observed Behavior: When a mob dies on top of a sculk catalyst, the bloom will not expand to blocks at grade or underneath the catalyst, unless it first starts from above the catalyst.
Testing: I tested this by dropping skeletons onto the catalysts, and placing blocks at different heights next to the catalyst.
In the image, this was accomplished by placing the catalyst: above, inside, and below the ice. Only the catalyst placed below the ice bloomed from mob death directly on top of the catalyst.
Created Issue:
Sculk Catalyst limited blooming when death is on top of Catalyst
Expected behavior: Sculk blooming to blocks under and around (at level) the sculk catalyst when mobs die on top of sculk catalyst.
Observed Behavior: When a mob dies on top of a sculk catalyst, the bloom will not expand to blocks at grade or underneath the catalyst, unless it first starts from above the catalyst.
Testing: I tested this by dropping skeletons onto the catalysts, and placing blocks at different heights next to the catalyst.
In the image, this was accomplished by placing the catalyst: above, inside, and below the ice. Only the catalyst placed below the ice bloomed from mob death directly on top of the catalyst.

In your own test, this is working as intended. See this comment by [Mojang] Gnembon
Basically; sculk charges can only travel through sculk. Since the top face of the catalyst is considered sculk, it is able to spread to blocks directly up and over the catalyst, and then further to other blocks. This video shows what I mean:
2023-11-06_15-35-49.mp4
(Notice how the sculk doesn't spread until the veins allow it to)