What type of bug can there be?

This is not because something is simple that it is weak

in anyway, browser based mnemonic generators, create seed phrase. I can't see why some seed phrases are less secure than other.

For example,I could write the 2048 words each one on piece of paper. Put all in a bag, then I pick 23 times a piece of paper by placing back the paper inside the bag each time. Between each picking, I shake the bag several minutes.
I do the same to get the 24th word by taking into account the checksum

By the way is there a way to measure the randomness with which a seed phrase was generated ?