I'm using:
fabric
Minecraft 1.20.4
MultiMC5 0.7.0-3863 (it says it's the latest version)

.minecraft\config\c3blocks.toml

for reference, I used this mod on an earlier version of Minecraft, sorry I don't remember which, and I had no issues editing the toml file and adding new blocks.

In my current version, I have tried editing the toml in vscode while the game is running, with the game closed. I have also tried from the mod-menu in-game, this seemed promising in real time.

No matter what I do--remove all blocks, add one block, add 100 blocks--the next time I open or close the game the toml file is reset to the default.

I have no idea what I'm doing wrong, or what interaction I may be missing. There is no error in the startup log, that I see.

contents before starting Minecraft (verified save)

MaxCompressionLevel = 9
COMPRESSED_BLOCKS = ["minecraft:stone", "minecraft:dirt", "minecraft:sand", "minecraft:diorite", "minecraft:andesite", "minecraft:tuff", "minecraft:deepslate", "minecraft:granite", "minecraft:cobbled_deepslate"]

contents after starting Minecraft (reopened)

MaxCompressionLevel = 9
COMPRESSED_BLOCKS = ["minecraft:stone", "minecraft:dirt", "minecraft:sand", "minecraft:diorite", "minecraft:andesite", "minecraft:tuff", "minecraft:deepslate", "minecraft:granite"]

I coppied the log from MultiMC. I'm sorry I'm your one issue here.
MultiMC_MC-1.20.4_C3-toml-reset.log

Let me know what else you may need