I've been mapping songs from anidb to mal anime ids, and discovered something about links to anidb entries. Some of them do not appear in manami, which is not the issue.
However, there seems to be an improvement potential, unmapped anidb anime titles all seem to be marked 18+. The thing is, anidb does not show them unless you are logged in and confirm you are over 18 y.o.
Here are some of those anidb ids for reference:
manami missing ids {'1640', '1847', '1166', '1278', '1546', '2803', '9549', '5358', '1492', '11194', '1426', '6002', '6765', '1876', '2748', '11182', '15258', '14102', '7984', '13384', '1556', '8814', '4544', '1844', '12157', '1522', '249', '6189', '1212', '1626', '8911', '4848', '5633', '1557', '307', '2566', '14252', '1629', '4981', '3289', '3982', '2747', '2552', '308', '2032', '5543', '13170', '4735', '2029', '7929', '2148', '1982', '2485', '2565', '549', '5203', '5359', '1333', '1328', '6484', '6817', '300', '9276', '851', '9469', '6561', '1088', '1396', '663', '664', '10942', '9699', '7259', '4347', '4443', '13652', '10504', '6208', '8994', '677', '4423', '1405', '174', '119', '15697', '12005', '7037', '907', '13761', '10263', '14340', '1047', '13411', '8262', '11085', '9933', '1603', '1987', '5594', '10296', '15510', '4078', '2254', '8402', '2141', '2145', '2571', '1875', '12547', '1624', '2573', '8473', '13550', '5205', '1628', '13285', '5541', '1627', '10964', '12512', '559', '12699', '700', '2567', '5031', '11047', '7030', '933', '1062', '1042', '12596', '1974', '849', '13489', '1168', '1420', '1510', '6154', '3527', '2373', '13356', '1843', '7915', '5200', '3999', '4493', '7489', '6675', '11476', '1338', '5362', '802', '1347', '8795', '2027', '10893', '1410', '10793', '7303', '2570', '267', '1642', '13251', '15630', '1108', '1720', '10839', '6781', '839', '1970', '8837', '1665', '4304', '2515', '3452', '1752', '2502', '2563', '6554', '667', '3531', '13834', '1337', '357', '1547', '7519', '1133', '1509', '5848', '7566', '243', '4155', '8508', '1159', '2268', '3537', '10221', '5889', '2804', '1346', '1694', '4075', '1207', '1331', '985', '5912', '5836', '2561', '870', '5193', '1433', '4823', '2701', '10189'}