Very interesting. As verification, it is handled as a case in "AdjustRecoveryTime" for CharacterStats (in the dlls via IlSpy). Awkwardly, I can't find a check anywhere for if there character is wearing a shield. As such, this may apply even if using a shield but with one weapon. The weird appearance is probably due to the entry for "SingleWeaponRecoveryModifier" in the status effect manager having a display string and tactical string of -1. Which is fixable in statuseffects.gamedatabundle, thankfully.
{
"StatusEffectType": "SingleWeaponRecoveryMultiplier",
"DataType": "4097",
"OperatorType": "Add",
----> "DisplayString": -1, <----
"WildcardString": -1,
"TacticalString": -1,
"AttackFilterDisplayStyleID": "2187499c-ae27-41cd-a373-74cfe55fa59f",
"AttackTargetFilterDisplayStyleID": "e3c2a249-5355-4d0f-a9e6-ff9441c76e0c"
},
{
"StatusEffectType": "DualWieldWeaponRecoveryMultiplier",
"DataType": "4097",
"OperatorType": "Multiply",
"DisplayString": 4040,
"WildcardString": -1,
"TacticalString": 5233,
"AttackFilterDisplayStyleID": "2187499c-ae27-41cd-a373-74cfe55fa59f",
"AttackTargetFilterDisplayStyleID": "e3c2a249-5355-4d0f-a9e6-ff9441c76e0c"
},