It will, as the key generation scheme used by Electrum differs from bare BIP39 https://electrum.readthedocs.io/en/latest/seedphrase.html Right now client is in position to import BIP 39 SEED but says that option can not be guaranteed one in the future. You can check it.
backward compatibility means when a user is capable of importing his BIP-39 mnemonic in Electrum today and the wallet file stores it, then it must continue being able to read BIP-39 mnemonics and convert them to child keys otherwise the old user's wallet can break down.
could you quote the exact part from the doc where it says that? i honestly cant see it.
Well, my 3.3.8 client is in Russian. So I will translate for you what is says. (Bold sentence is a key point.)
Attention. BIP39 seed phrases can be imported into Electrum for user access to funds from other wallets. We do not generate seed-phrase in BIP39 format since This does not meet our safety standard. However, BIP39 seed phrases do not include a version number, which compromises compatibility with future software wallets. We cannot guarantee that BIP39 encryption will always be supported by Electrum.