Really? I get doing it for certain parts like the BLACKPINK!! chant but they had Lisa's entire rap playing in the background while she was also rapping, it was weird.
Whenever a group is performing, the vocals of the original song is playing in the background. Depending on the show's decision, it's either really low or loud. Groups truly lip-syncing the entire song isn't really common. But if you don't hear them properly, it's either because the song in the background is too loud and is overshadowing their voice, or they decide to not sing specific parts live. It's really common. Especially when it's for Music Shows and Performances like this.
Even for rapping, it's something that happens often. Rappers like Mino, Bobby, Hanbin, Tablo etc. even like to skip certain parts of their rap and you will hear the song playing in the background.