I set up a proxy in vite.config.ts:

import { fileURLToPath, URL } from 'node:url'

import { defineConfig } from 'vite'
import vue from '@vitejs/plugin-vue'
import vueJsx from '@vitejs/plugin-vue-jsx'
import VueDevTools from 'vite-plugin-vue-devtools'

// https://vitejs.dev/config/
export default defineConfig({
  plugins: [
    vue(),
    vueJsx(),
    VueDevTools(),
  ],
  resolve: {
    alias: {
      '@': fileURLToPath(new URL('./src', import.meta.url))
    }
  },
  server: {
    proxy: {
      '/api': {
        target: 'http://localhost:5000',
        changeOrigin: true,
        rewrite: path => path.replace(/^/api/, ''),
      },
    },
  }
})

This is my app.ts code looks like:

app.post('/login', async (req, res) => {
    try {
        const { username, email, password } = req.body;
        console.log(username, email, password);
        let user = await User.findOne({ $or: [{ username }, { email }] });
        if (!user) {
            return res.status(400).json({ message: 'User not found' });
        }
        const isPasswordMatch = await user.comparePassword(password);
        if (!isPasswordMatch) {
            return res.status(400).json({ message: 'Invalid password' });
        }
        const userInfo = { id: user._id, username: user.username, email: user.email };
        const token = jwt.sign({ userId: user._id }, process.env.JWT_SECRET!, { expiresIn: '1h' });
        res.cookie('token', token, { httpOnly: true, maxAge: 24 * 60 * 60 * 1000 });
        res.status(200).json({ message: 'Login successful', user: userInfo });
    } catch (error) {
        res.status(500).json({ message: 'Login failed', error });
    }
});

const storage = multer.memoryStorage();
const upload = multer({ storage: storage });

app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));

app.post('/newpost', upload.array('media'), async (req, res) => {
    try {
        const { username, caption, price, status, location } = req.body;
        const media = (req.files as Express.Multer.File[]).map(file => ({
            media: file.buffer.toString('base64')
        }));
        const newPost = new Post({ username, caption, price, status, location, uploaded: media });
        await newPost.save();
        res.status(201).json({ message: 'Post created successfully', post: newPost });
    } catch (error) {
        res.status(500).json({ message: 'Post creation failed', error });
    }
});

My axios.post in this function works fine:

const postNewPost = async () => {
    const formData = new FormData();
    formData.append('username', 'testuser');
    formData.append('caption', caption.value);
    formData.append('status', selectedStatus.value);
    formData.append('price', price.value?.toString() || '');
    formData.append('location', location.value);
    
    selectedMedia.value.forEach((file) => {
        formData.append('media', file);
    });
    try {
        const response = await axios.post('/api/newpost', formData, {
            headers: {
                'Content-Type': 'multipart/form-data',
            },
        });
        alert('Post created successfully!');
        emit('post-created');
    } catch (error) {
        console.error(error);
    }
};

But it does not work in this function:

const loginUser = async () => {
    const formData = new FormData();
    const { value: usernameEmailValue } = usernameEmail;
    if (emailRegex.test(usernameEmailValue)) {
        formData.append('email', usernameEmailValue);
    } else {
        formData.append('username', usernameEmailValue);
    }
    formData.append('password', passwordLogin.value);
    try {
        const response = await axios.post('/api/login', formData);
        if (response.status === 200) {
            alert('Login successful');
        } else {
            alert('Login failed');
        }
    } catch (error) {
        console.error('Login failed:', error);
    }
};

when it fails to follow the proxy rule. In console, it says:
POST http://localhost:5173/api/login 500 (Internal Server Error)

Why does this happen even though these functions look almost the same?