It seems my original post got eaten.
I have a Ranger with the Horde Breaker feature. He has been in situations where he's in the middle of three creatures that are all one square away from each other, and his bonus horde breaker attack has not triggered. However, he has also been in situations where he is using a bow to attack creatures that are presumably one square away from each other, and he gets a bonus ranged attack. He's level 4, so no extra attacks have been gained that way. The behaviour of the feature appears to need a bit of tuning.
Et quod est superius est sicut quod est inferius, ad perpetranda miracula rei unius.
I cannot help but notice what you describe is the Marksman subclass feature (ranged Reaction to a ranged attack within 10 feet).
If you double check your character, does your Ranger really have Horde Breaker ? If so, it looks like the two features get crisscrossed in some of the coding.
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Apologies - I was mistaking the 5e Horde Breaker (bonus attack when in base contact with more than one enemy) with the Solasta one (extra attack when you kill an enemy)
Et quod est superius est sicut quod est inferius, ad perpetranda miracula rei unius.