From what I've learned about Electrum, it uses it's own standard to generate seed phrases.  Although it may be based on Bip39, it's not 100% compatible with Bip39.  I'm not the right guy to divulge the technical details, but I believe Electrum includes a checksum that helps the software determine if the seed is for a legacy wallet, or a native segwit wallet.

The only way I know of to generated p2sh addresses using an Electrum generated seed is to trick the software into believing it's a Bip39 seed.  The more common method for generating p2sh addresses is by using an actual Bip39 seed, like one that was generated by Ian Coleman's tool or a hardware wallet.