Looks like it’s hard to associate LVM group with physical volumes created by Gparted when running LMDE 6 installation vs GUI provided by classical Debian Network installer. Either I am missing some technical skills or LMDE 6 just cannot provide standard functionality of Debian Network Installer has been known for a while. Say Debian 10/11/12 at least as far as to my knowledge .

Source: LXer – LMDE Faye (6) vs Debian (11/12) network installers regarding LVM flexibility