Let Me Solo Her, the premier expert on being summoned to fight Malenia, has posted a video of this bug from patch 1.04.
Around the 8:30 timestamp in the video, you can see Let Me Solo Her fighting Malenia, and it’s quite a battle as she continually heals, even while Let Me Solo Her flawlessly dodges all of her attacks.
Waterfowl Dance is a little too strong Malenia is arguably the toughest boss in Elden Ring, and a major part of her difficulty is that she can heal herself by hitting you with her sword attacks.
FromSoftware have been patching Elden Ring since the game’s launch; there have been numerous bug fixes in recent patches, including one that nerfed its extremely scary blood dogs.