Jetz and I confirmed this report. Visually, Zerek indicates that he will be resummoned. Additionally, the buff is added to indicate that he will be resummoned. It seems like the visual indicators are added when the spell is cast but the deathrattle additionally checks that the spell resolved, causing it to not trigger.
I was playing Priest in a Priest-Rogue game - the Rogue had an empty board, and I had an unbuffed copy of Zerek in play; Rogue played a mage secret and passed the turn.
I attempted to buff Zerek with Extra Arms and triggered Spellbender; the token received +2/+2 as expected, but Zerek was also buffed to indicate he would trigger his deathrattle. Ultimately, he died and did not resummon (as should be expected) despite the visual indicator and tooltip to the contrary.
Jetz and I confirmed this report. Visually, Zerek indicates that he will be resummoned. Additionally, the buff is added to indicate that he will be resummoned. It seems like the visual indicators are added when the spell is cast but the deathrattle additionally checks that the spell resolved, causing it to not trigger.
https://us.forums.blizzard.com/en/hearthstone/t/spellbender-and-zerek-visual-bug/3313
Mage AAEBAf0EAu0Fl8ECDrMB+wG7Au4CvwOrBLQEywSWBbkGt2zBwQLiygKm8AIA
Priest AAEBAaCsAwLL+AKLigMOlwKcAvgC5QS5Bo0PiBSWFNLBAqbwAtD+AvGAA+qMA8+dAwA=