Function not fully running from advancement reward, but runs normally when using `/function`

Hello!
For my modpack, I need to run some commands on world creation. I made an advancement with the minecraft:tick trigger, which runs my function. And for all I can tell, it runs the function partially (?)
Here is the function:
# Prevent the function from running on rejoin
# TODO does this actually work?
say "Executing function init..."
execute if data storage minecraft:world created run return 1
data modify storage minecraft:world created set value 1
say "Executing function init (for real this time)..."

# Place initial cube
say "Placing blocks..."
fill -10 -1 -10 10 20 10 minecraft:bedrock
fill -3 0 -3 3 6 3 minecraft:air
setblock 0 0 0 minecraft:torch
say "Placed blocks!"

# Give initial items
say "Giving items..."
clear @a
give @a minecraft:dirt
give @a minecraft:oak_sapling
give @a supplementaries:flax_seeds
say "Gave items!"

# Teleport player into the cube
say "Teleporting player..."
tp @a 0 0 0
setworldspawn 0 0 0
spawnpoint @a 0 0 0
say "Teleported player!"

Now what happens is that no blocks get placed, but I am given the starter items and am teleported to 0/0/0, ending up with me falling into the void (I use a void world preset). Curiously, when I use /function to run that exact same function, the blocks are placed as expected. I tried some things, including the say commands to verify it's actually running, and commenting out everything that isn't the fill and setblock commands, to no avail.