Summary:

Sculk veins can only spread on a vertical surface at the edges of the "lake", but not in the center (if they are not extreme blocks).
To do this, you need to use a surface that is not replaced by a sculk.

Steps to reproduce:

1. 
   Build the structure as shown in the image (the surface must be made of blocks that cannot be replaced by sculk).
2. Summon a mob in the center (such as a creeper).
3. Kill the mob.

Observed results:

Only the outermost blocks of sculk veins will spread vertically.

Expected results:

The sculk veins blocks can spread vertically without hindrance, even in the center of the "sculk lake".

Video:

Minecraft 2021-10-24 11-23-22.mp4

Image: