There's no way to see your BIP39 seed phrase from Electrum.
Unlike a native Electrum seed phrase which is saved in the wallet file under "keystore", BIP39's isn't saved there, just the derived extended private/public keys.
Without the actual seed phrase, it impossible to get it from the extended private key (xprv) that's derived from it.

The most probable reason is the developers' take on BIP39 that its seed phrase isn't enough as a backup since the user also has to take note of derivations path and script type.
As for actual reference, this doesn't have enough information to be a reference, but here is it anyways: github.com/spesmilo/electrum/issues/4602#issuecomment-409913017