针对issue#I4X9TT修复

hutool-core/src/main/java/cn/hutool/core/text/NamingCase.java

@@ -97,7 +97,7 @@ public class NamingCase {
 						// 后一个为大写，按照专有名词对待，如aBC -> a_BC
 					} else {
 						//前一个为大写
-						if (null == nextChar || Character.isLowerCase(nextChar)) {
+						if (null != nextChar && Character.isLowerCase(nextChar)) {
 							// 普通首字母大写，如ABcc -> A_bcc
 							sb.append(symbol);
 							c = Character.toLowerCase(c);

hutool-core/src/test/java/cn/hutool/core/util/StrUtilTest.java

@@ -422,6 +422,7 @@ public class StrUtilTest {
 				.set("H2", "H2")
 				.set("H#case", "H#case")
 				.set("PNLabel", "PN_label")
+				.set("DEPT_NAME","DEPT_NAME")
 				.forEach((key, value) -> Assert.assertEquals(value, StrUtil.toUnderlineCase(key)));
 	}