1. Lots of things get discussed. This was such a minor topic that Gorbachev didn't even recall it ever having been on agenda. Ultimately discussions lead to a formal agreement with a text approved by all sides and that's what counts. There is no formal agreement, only tiny snippets that mention it ever having been discussed.
2. The Soviet Union doesn't exist anymore. Even if such assurances had been made, a number (most by now?) of former parts of the Soviet Union have stepped away from it and applied for NATO. Why should Lithuania be eternally bound to an assurance made to the Soviet delegation, which at the time represented Lithuania? Even if all former members of the USSR applied for NATO, should they be denied because of the alleged assurance in the past to an entity that no longer exists?
2. The Soviet Union doesn't exist anymore. Even if such assurances had been made, a number (most by now?) of former parts of the Soviet Union have stepped away from it and applied for NATO. Why should Lithuania be eternally bound to an assurance made to the Soviet delegation, which at the time represented Lithuania? Even if all former members of the USSR applied for NATO, should they be denied because of the alleged assurance in the past to an entity that no longer exists?