Do you have a screencap of this occurring? The combat log, your character sheet and potential damage, the enemy statistics, etc? Attaching that stuff would possible help diagnose the issue.
The game will almost certainly have a Story Mode difficulty on full release.
To be clear, that rule liked is not an official 5e Variant Rule. It is a completely-homebrewed rule, as is most everything on dandwiki.com (in addition to being very unbalanced), so it's very unlikely that Larian would implement it. It's more likely that said enemy is (incorrectly/accidentally?) using the actual
rule for damage resistance which is half damage. If you inspect said halfling, does it have any type of damage resistance--e.g., slashing--listed? As DR rounds down, 3 damage would become 1.
