Having the Prosecutor General, who is responsible for their own prosecution, attend a Cabinet meeting?

This is highly inappropriate behavior.
If you are unsure, just try replacing Lee Jae-myung with Yoon Suk-yeol and ask yourself, "Is this allowed?" and you'll have your answer.
If that is too difficult, just imagine Kim Keon-hee being acquitted, winning the presidential election, and then sitting the Prosecutor General, the head of the Central Investigation Agency, and the Commissioner General of the Police at a Cabinet meeting, playing around with them while claiming there are no constitutional prohibitions.