Try this.
More like a tempfix, it should work.
Index: java/org/l2jmobius/gameserver/network/clientpackets/EnterWorld.java
===================================================================
--- java/org/l2jmobius/gameserver/network/clientpackets/EnterWorld.java (revision 8275)
+++ java/org/l2jmobius/gameserver/network/clientpackets/EnterWorld.java (working copy)
@@ -79,6 +79,7 @@
import org.l2jmobius.gameserver.network.serverpackets.ItemList;
import org.l2jmobius.gameserver.network.serverpackets.MagicSkillUse;
import org.l2jmobius.gameserver.network.serverpackets.NpcHtmlMessage;
+import org.l2jmobius.gameserver.network.serverpackets.PartySmallWindowDeleteAll;
import org.l2jmobius.gameserver.network.serverpackets.PlaySound;
import org.l2jmobius.gameserver.network.serverpackets.PledgeShowMemberListAll;
import org.l2jmobius.gameserver.network.serverpackets.PledgeShowMemberListUpdate;
@@ -478,6 +479,9 @@
// Send all skills to char
player.sendSkillList();
+ // Remain in party after logout fix.
+ player.sendPacket(new PartySmallWindowDeleteAll());
+
// Close lock at login
player.setLocked(false);
}