Just had the same experience in two duels in a row, played Shadow word: void on a 1-health and 2-health minion, expecting it to summon one Voidfiend. Instead, it had no effect. Both spells were also duplicated on other one-health minions at the end of my turn to no effect.
The ‘rounded up’ in the card text makes me think I should summon one Voidiend in both circumstances.
Your battlefield only has room for a total of seven minions, dormant minions, and locations.
Was it already full?
No, plenty of room (one active minion in the first instance, two in the second). Should have grabbed screen recording but didn’t think to report until later.
I can confirm it behaves like this, always drop a wow emote when an opponent plays it and nothing summons but forget to report it because I am easily sidetracked.