Weird bug with buckler
 

Hello everyone.

I was building a bard and everything was going fine, but when I gave her a rapier and a buckler her Attack Bonus Calculation went down by one point, stating a "-1 (ACP from shield)". From what I know bards should be proficient with bucklers, and even the archetype I'm using (Chelish diva) has that proficiency; then why is HLC calculating that malus as if my character wasn't proficient?

Here's the full statblock:

Stelia Imperiosus
Male devil-spawn tiefling bard (chelish diva) 1 (Pathfinder Player Companion: Blood of Fiends 21, Pathfinder Campaign Setting: Inner Sea Magic, Pathfinder RPG Bestiary 264)
NG Medium outsider (native)
Init +3; Senses darkvision 60 ft.; Perception +4
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Defense
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AC 16, touch 13, flat-footed 13 (+2 armor, +3 Dex, +1 shield)
hp 10 (1d8+2)
Fort +2, Ref +5, Will +2
Resist cold 5, electricity 5, fire 5
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Offense
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Speed 30 ft.
Melee dagger +3 (1d4+1/19-20) or
. . rapier +2 (1d6+1/18-20)
Ranged dagger +3 (1d4+1/19-20) or
. . shortbow +3 (1d6/×3)
Special Attacks bardic performance 6 rounds/day (countersong, distraction, fascinate [DC 12], inspire courage +1)
Bard (Chelish Diva) Spells Known (CL 1st; concentration +3)
. . 1st (2/day)—cure light wounds, sleep (DC 13)
. . 0 (at will)—dancing lights, lullaby (DC 12), mending, prestidigitation
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Statistics
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Str 12, Dex 16, Con 14, Int 11, Wis 10, Cha 15
Base Atk +0; CMB +3; CMD 14
Feats Agile Maneuvers, Deceitful
Traits charming, diva in training, vagabond child (urban)
Skills Bluff +8 (+6 for 24 hours when you fail an opposed Charisma based check, +9 vs. characters who could be attracted to you), Diplomacy +8 (+6 for 24 hours when you fail an opposed Charisma based check, +9 vs. characters who could be attracted to you), Disguise +8 (+6 for 24 hours when you fail an opposed Charisma based check), Intimidate +2 (+0 for 24 hours when you fail an opposed Charisma based check), Knowledge (local) +4, Perception +4, Perform (sing) +7 (+5 for 24 hours when you fail an opposed Charisma based check), Sense Motive +6, Sleight of Hand +7, Stealth +6; Racial Modifiers +2 Diplomacy, +2 Sense Motive
Languages Common
SQ famous, finesse weapon attack attribute, pass for human, vain
Other Gear leather armor, buckler, arrows (20), dagger, rapier, shortbow, backpack, bedroll, belt pouch, flint and steel, hemp rope (50 ft.), ink, inkpen, journal[UE], mess kit[UE], mirror, pot, soap, torch (10), trail rations (5), waterskin, 1 gp
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Special Abilities
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Agile Maneuvers Use DEX instead of STR for CMB
Bardic Performance (standard action, 6 rounds/day) Your performances can create magical effects.
Darkvision (60 feet) You can see in the dark (black and white only).
Energy Resistance, Cold (5) You have the specified Energy Resistance against Cold attacks.
Energy Resistance, Electricity (5) You have the specified Energy Resistance against Electricity attacks.
Energy Resistance, Fire (5) You have the specified Energy Resistance against Fire attacks.
Famous +1 (Jarvis End, Kintargo) With natives of selected region gain bonus to Bluff & Intimidate.
Finesse Weapon Attack Attribute Finesse weapons use Dexterity on attack rolls.
Pass for Human You can pass for human, and count as a humanoid (human) for all purposes.
Vain -2 to Cha checks for 24h after failing an opposed Cha check

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Thank you in advance for your help and time.

Do you have a weapon in your off-hand? You have a -1 penalty on attack rolls while a weapon is equipped in your off-hand when using a buckler.

I have a rapier in my main hand and the buckler in my offhand, nothing else.

It's Weapon Finesse:
"Benefit: With a light weapon, elven curve blade, rapier, whip, or spiked chain made for a creature of your size category, you may use your Dexterity modifier instead of your Strength modifier on attack rolls. If you carry a shield, its armor check penalty applies to your attack rolls."