No it does not save the mnemonic seed phrase when you create a new descriptor wallet.
Those descriptor wallet use xpub/xprv (extended public/private keys) to generate addresses and manage keys.
However, if you have the master private key (xprv), you can still recover your wallet and funds without the mnemonic seed phrase.
The reason why Bitcoin Core does not save the mnemonic seed phrase when creating a new descriptor wallet is for security reasons, it could be compromised because the seed key is the master key to the wallet and so an attacker could potentially gain access to all its fund. The xprv is effectively derived from the seed phrase, and so it is not necessary to save the seed phrase in order to recover the wallet