Vanar had 2 Freeblade sentinels, after summoning a Lavaslaher to kill one sentinel, it transformed into a full health Freeblade the other sentinel also turned into a Freeblade. The first one which was punched by Lavaslasher switches positions with the Lavaslaher, then the Lavaslasher goes to the position of the second Freeblade, but the second Freeblade goes into the spot of the first Freeblade, meaning the 2 Freeblades turns into 1, and that Freeblade was then immune to Plasma Storm.
Edit: After rewatching what happened, it seems that the Freeblades were on top of eachother, 1 died to Plasma Storm, and the second popped up still alive in its place.