Tadatsugu enjoyed great confidence of Ieyasu, and when he was inquired by Nobunaga ODA about Nobuyasu MATSUDAIRA, a legitimate child of Ieyasu, in 1579, Tadatsugu was designated as an envoy for defense and went to Azuchi Castle together with Tadayo OKUBO, but he was not able to defend Nobuyasu well enough and thus failed to prevent the order for Nobuyasu's seppuku (suicide by disembowelment) from being given (though there is a different view on the claim that the incident of Nobuyasu's seppuku was under the order of Nobunaga).
家康の厚い信任を受けていた忠次ではあったが、天正7年（1579年）に家康の嫡子・松平信康の件で織田信長からの詰問を受けたとき、大久保忠世とともに弁解の使者に立てられて安土城に赴いたものの、信康を十分に弁護せず信康切腹の命令を防げなかった（ただし、信康切腹事件が信長の命令によるという件は、異説も存在する）。