diff --git a/packages/vue-i18n-core/src/composer.ts b/packages/vue-i18n-core/src/composer.ts
index df29a5c4e..4a24ecd9c 100644
--- a/packages/vue-i18n-core/src/composer.ts
+++ b/packages/vue-i18n-core/src/composer.ts
@@ -11,7 +11,8 @@ import {
   isPlainObject,
   isObject,
   assign,
-  inBrowser
+  inBrowser,
+  hasOwn
 } from '@intlify/shared'
 import {
   isTranslateFallbackWarn,
@@ -53,7 +54,8 @@ import {
   deepCopy,
   getLocaleMessages,
   getComponentOptions,
-  createTextNode
+  createTextNode,
+  handleFlatJson
 } from './utils'
 import { VERSION } from './misc'
 
@@ -1848,6 +1850,7 @@ export function createComposer(options: any = {}, VueI18nLegacy?: any): any {
     NumberFormatsType
   >
   const _isGlobal = __root === undefined
+  const flatJson = options.flatJson
 
   let _inheritLocale = isBoolean(options.inheritLocale)
     ? options.inheritLocale
@@ -2375,6 +2378,15 @@ export function createComposer(options: any = {}, VueI18nLegacy?: any): any {
 
   // setLocaleMessage
   function setLocaleMessage(locale: Locale, message: LocaleMessage<Message>) {
+    if (flatJson) {
+      const _message = { [locale]: message }
+      for (const key in _message) {
+        if (hasOwn(_message, key)) {
+          handleFlatJson(_message[key])
+        }
+      }
+      message = _message[locale]
+    }
     _messages.value[locale] = message
     if (__BRIDGE__) {
       __legacy && __legacy.setLocaleMessage(locale, message)
@@ -2391,6 +2403,13 @@ export function createComposer(options: any = {}, VueI18nLegacy?: any): any {
     if (__BRIDGE__) {
       __legacy && __legacy.mergeLocaleMessage(locale, message)
     }
+    const _message = { [locale]: message }
+    for (const key in _message) {
+      if (hasOwn(_message, key)) {
+        handleFlatJson(_message[key])
+      }
+    }
+    message = _message[locale]
     deepCopy(message, _messages.value[locale])
     _context.messages = _messages.value as typeof _context.messages
   }
diff --git a/packages/vue-i18n-core/test/issues.test.ts b/packages/vue-i18n-core/test/issues.test.ts
index 6b31b691a..83a4900f2 100644
--- a/packages/vue-i18n-core/test/issues.test.ts
+++ b/packages/vue-i18n-core/test/issues.test.ts
@@ -1049,3 +1049,119 @@ test('issue #1547', async () => {
 
   expect(wrapper.html()).toEqual('<div>Deep Linked message</div>')
 })
+
+test('issue #1595', async () => {
+  const i18n = createI18n({
+    legacy: false,
+    locale: 'en',
+    flatJson: true,
+    messages: {
+      en: {
+        simple: 'Simple',
+        'deep.key': 'Deep',
+        content: '@:simple @:deep.key'
+      }
+    }
+  })
+
+  const ja = {
+    simple: 'シンプル',
+    'deep.key': 'ディープ',
+    content: '@:simple @:deep.key'
+  }
+  i18n.global.setLocaleMessage('ja', ja)
+
+  const App = defineComponent({
+    setup() {
+      const { t, locale } = useI18n()
+      return { t, locale }
+    },
+    template: `<form>
+  <select v-model="locale">
+    <option value="en">en</option>
+    <option value="ja">ja</option>
+  </select>
+</form>
+{{ t('content') }}
+`
+  })
+
+  expect(i18n.global.getLocaleMessage('ja')).toEqual({
+    simple: 'シンプル',
+    deep: {
+      key: 'ディープ'
+    },
+    content: '@:simple @:deep.key'
+  })
+
+  const wrapper = await mount(App, i18n)
+  expect(wrapper.html()).toEqual(
+    '<form><select><option value="en">en</option><option value="ja">ja</option></select></form> Simple Deep'
+  )
+  // @ts-ignore
+  i18n.global.locale.value = 'ja'
+  await nextTick()
+
+  expect(wrapper.html()).toEqual(
+    '<form><select><option value="en">en</option><option value="ja">ja</option></select></form> シンプル ディープ'
+  )
+})
+
+test('issue #1595 merge case', async () => {
+  const i18n = createI18n({
+    legacy: false,
+    locale: 'en',
+    flatJson: true,
+    messages: {
+      en: {
+        simple: 'Simple',
+        'deep.key': 'Deep',
+        content: '@:simple @:deep.key'
+      },
+      ja: {
+        simple: 'シンプル',
+        content: '@:simple @:deep.key'
+      }
+    }
+  })
+
+  const ja = {
+    'deep.key': 'ディープ'
+  }
+  i18n.global.mergeLocaleMessage('ja', ja)
+
+  const App = defineComponent({
+    setup() {
+      const { t, locale } = useI18n()
+      return { t, locale }
+    },
+    template: `<form>
+  <select v-model="locale">
+    <option value="en">en</option>
+    <option value="ja">ja</option>
+  </select>
+</form>
+{{ $t('content') }}
+`
+  })
+
+  expect(i18n.global.getLocaleMessage('ja')).toEqual({
+    simple: 'シンプル',
+    deep: {
+      key: 'ディープ'
+    },
+    content: '@:simple @:deep.key'
+  })
+
+  const wrapper = await mount(App, i18n)
+  expect(wrapper.html()).toEqual(
+    '<form><select><option value="en">en</option><option value="ja">ja</option></select></form> Simple Deep'
+  )
+  // @ts-ignore
+  i18n.global.locale.value = 'ja'
+  await nextTick()
+
+  expect(wrapper.html()).toEqual(
+    '<form><select><option value="en">en</option><option value="ja">ja</option></select></form> シンプル ディープ'
+  )
+})