diff --git a/src/case.ts b/src/case.ts
index 4cd770126..ec6d5a320 100644
--- a/src/case.ts
+++ b/src/case.ts
@@ -6,12 +6,9 @@ export function maybeSnakeToCamel(s: string, options: Pick<Options, 'snakeToCame
     return s
       .split('_')
       .map((word, i) => {
-        if (i === 0) {
-          // if first symbol is "_" then skip it
-          return word ? word[0] + (hasLowerCase ? word.substring(1) : word.substring(1).toLowerCase()) : '';
-        } else {
-          return capitalize(word.toLowerCase());
-        }
+        // If the word is already mixed case, leave the exist case as-is
+        word = hasLowerCase ? word : word.toLowerCase();
+        return i === 0 ? word : capitalize(word);
       })
       .join('');
   } else {
diff --git a/tests/case-test.ts b/tests/case-test.ts
index 1e5a55764..37dd06b74 100644
--- a/tests/case-test.ts
+++ b/tests/case-test.ts
@@ -13,11 +13,13 @@ describe('case', () => {
   });
 
   it('de-upper cases', () => {
-    expect(maybeSnakeToCamel('FOO_BAR', keys)).toEqual('FooBar');
+    expect(maybeSnakeToCamel('FOO_BAR', keys)).toEqual('fooBar');
   });
 
   it('leaves existing mixed cases', () => {
     expect(maybeSnakeToCamel('clientI_d', keys)).toEqual('clientID');
+    expect(maybeSnakeToCamel('menu_calendarI_d', keys)).toEqual('menuCalendarID');
+    expect(maybeSnakeToCamel('display_nameI18n', keys)).toEqual('displayNameI18n');
   });
 
   it('leaves the first character as it was', () => {