Blocks in Volto

I was using Volto 12.5.0 and following https://training.plone.org/5/voltohandson/introtoblocks.html

Now while creating new block ie custom block suggested approach was while registering in config

export const blocks = {
  ...defaultBlocks,
  blocksConfig: { ...defaultBlocks.blocksConfig, ...customBlocks },
};

but this did not cause any changes so I had to directly change defaultBlocks object

defaultBlocks.blocksConfig = { ...defaultBlocks.blocksConfig, ...customBlocks };

This made the new block available.
Need help in understanding why the default exported blocks object is not causing any changes, or am I doing something wrong.

You didn't do anything wrong, the problem is that with Volto 12 the configuration system has changed and the training hasn't been updated. The authoritative documentation for Volto is at https://docs.voltocms.com/ (although you may find issues there, as well).

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