Default language subdirectory breaks hyperlinks

Saw an old post regarding the same issue. However, it was marked resolved without providing a resolution.

here is the tread Default language subdirectory breaks hyperlinks | www.remarpro.com

Problem Description:

We enabled “Use a subdirectory for the default language” and moved the preferred language to the top of the languages list.

The zh_tw works find

old:?https://domainname/gallery?new:?https://domainname/zh_tw/gallery
old:?https://domainname/about-us?new:?https://domainname/zh_tw/about-us

This is exactly what we want and all the URLs work as expected.

For the English version of our website, however, all the URLs are unfortunately broken. Again, sticking to the same example as before. Here’s what the URLs are for the English version of the website:
old:?https://domainname/gallery?new:?https://domainname/en_us/1
old:?https://domainname/about-us?new:?https://domainname/en_us/1

The browser returns 404
Please help.

Thank you