I found a way to fix the bug When you are in fighting Miraak and he becomes invincible. there is a way to fix it.(either relonikiiv or kruzikrel must still be alive).

when the first dragon is killed Mirak may become invincible. what you need to do is mount sahrotar and get him to kill the other living dragon, which causes miraak to absorb the soul and start fighing again. after he absorbs the soul you must dismount the sahrotar. fight miraak until sahrotar is killed by miraak then fight him again.

this bug happened to me yesterday and I retried 14 times with no luck. I then had 1 more attempt but Miraak was still invincible. i then done All the steps above and it worked.

this bug is really immersion breaking and game breaking, im end up punching mirrak to death just because of this bug =.=

well...surely this shows how pathetic the boss is