incorrect error message: Cannot delete rows from table which is parent in a foreign key constraint

CREATE TABLE `Tab1` (

  `T1Id` int(11) NOT NULL,

  `T1ExtId` varchar(60) NOT NULL,

  PRIMARY KEY (`T1Id`),

  UNIQUE KEY `UIdxTab1T1ExtId` (`T1ExtId`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;

CREATE TABLE `Tab3` (

  `T3Id` int(11) NOT NULL,

  `T3Str` varchar(60) NOT NULL,

  `T3Num` int(11) NOT NULL,

  `T1Id` int(11) NOT NULL,

  PRIMARY KEY (`T3Id`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;

CREATE TABLE `Tab4` (

  `T4Id` int(11) NOT NULL,

  `T4Str` varchar(60) NOT NULL,

  `T3Id` int(11) NOT NULL,

  PRIMARY KEY (`T4Id`),

  KEY `FkTab4T3Id` (`T3Id`),

  CONSTRAINT `FkTab4T3Id` FOREIGN KEY (`T3Id`) REFERENCES `Tab3` (`T3Id`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;

insert into Tab1 (T1Id, T1ExtId) values (1, 'one'), (2, 'two');

insert into Tab3 (T3Id, T3Str, T3Num, T1Id) values (1, 'one', 1, 1), (2, 'two', 11, 2), (3, 'three', 1111, 1), (4, 'four', 11111, 2);

insert into Tab3 (T3Id, T3Str, T3Num, T1Id) values (5, 'one', 1, 1);

alter table Tab3 

add constraint FkTab3T1Id foreign key (T1Id) references Tab1 (T1Id),

add unique index UIdxTab3T3Str (T3Str);

Cannot delete rows from table which is parent in a foreign key constraint 'FkTab4T3Id' of table 'Tab4'

alter table Tab3 add constraint FkTab3T1Id foreign key (T1Id) references Tab1 (T1Id);

alter table Tab3 add unique index UIdxTab3T3Str (T3Str);

Duplicate entry 'one' for key 'UIdxTab3T3Str'