I have experienced issues while trying to use multiple layers of blocks for a custom preset.
(It just doesn't show up after adding it)

So I looked around a bit and found that if you seperate the blocks with a comma (how the block parser should work) the regex parsing the whole string doesn't account for a comma.

The regex that is built in: ^(?<name>[a-zA-Z0-9_ -]+);(?<blocks>[a-z0-9_:\.\*-]+);(?<biome>[a-z0-9_:]+);(?<options>[a-z0-9_, \(\)=]*);(?<icon>[a-z0-9_:-]+(?:@[0-9]+)?)$
How the regex should be: ^(?<name>[a-zA-Z0-9_ -]+);(?<blocks>[a-z0-9_:\.\*-,]+);(?<biome>[a-z0-9_:]+);(?<options>[a-z0-9_, \(\)=]*);(?<icon>[a-z0-9_:-]+(?:@[0-9]+)?)$
(Note the comma in the "blocks" group at the end.)

I'm just guessing that that might be the issue, but I hope it gets fixed.