I've never seen the Desktop (Window, Linux, Mac) versions refer to "default_wallet" or attempt to create a wallet with that name.
I have no idea which version of Electrum I first used on my computer, but my first Electrum wallet (which I still have, albeit with no coins in it) is named "default_wallet". I don't think this has changed recently.
Certainly according to the documentation here (
https://electrum.readthedocs.io/en/latest/faq.html#where-is-my-wallet-file-located), the first wallet created is called "default_wallet".
Interesting. It's been months since I've played with the Linux version, and I avoid Macs like the centralized plague that they are (but my daughters use them.) On my Windows PCs Electrum always defaults to "wallet_1" as the name for the first wallet created.