First of all, electrum doesn't generate or imports 256 bits of entropy, but only 128, that's why it returns you only 12 words.
Electrum can generate a 24-word seed phrase, if you use console.
For generating a seed phrase with 256 bits of entropy, you can use the command below.
make_seed(256)
Or the following command if you want legacy addresses.
make_seed(256,"","standard")
After generating the seed phrase on console tab, you can create a new wallet with importing the 24-word seed.
