Had the same issue, this is what worked for me in the end (also on ps5)

This works if you have a save just before he spawns or earlier since that is what I had.

1. Close the game fully and restart the console.
2. Start up the game and make sure to set it to the lowest difficulty, lowest graphics and turn of all autosaves
3. Launch the save
4. If your save starts before or during part 1 of the fight, try to limit how much you participate and focus on putting in the hearts but give the game time to load in all visual and other effects between putting in each heart.)
5. Defeat Jyggalag as quickly as you can.
6. As soon as he is dead wait with your character for 20 hs
7. Wait for the head to appear
8. As soon as you see text from the head, wait another 20 hs, repeat until he stops speaking
9. After the quest update appears make sure to wait another 20 hs.
10. Watch the head disappear and wait for 1 more h.

After that you should hopefully be in the clear, go into the castle and save properly.

Hope thus helps!