When editing a post date with some locales, the date format of month would shows wrong one.

[tai]

When editing a post date with some locales, the date format of month would shows wrong one.

It happens only when using the classic editor functions in the core, and the Gutenberg editor shows the right date format.

Step to reproduce:

1. Install the classic editor and activate it.
2. Select "日本語" (Japanese) at "Site Language" option.
3. Open an edit screen of any post.
4. Now the Published date (in ja "投稿日") string looks like "2020年2月12日"(bug-1.png)
5. Click "編集"(Edit) at "投稿日" in Publish block.
6. Select any month of NOT current one and click the OK button.
7. Now the Published date string looks like "2020年1月月12日"(bug-2.png), inserted the unnecessary month suffix of Japanese "月".

How to fix:

I've dug into the source and found it happens around the below but no idea how to fix so far:
​https://github.com/WordPress/WordPress/blob/master/wp-admin/js/post.js#L762-L772

Version:
I've checked it on WordPress 5.4-beta3 and WordPress 5.3.2

[tai]

right month string.

[tai]

wrong month string.

[tai]

I've just confirmed it doesn't happen on WordPress 5.2.4, so I guess it was introduced after WordPress 5.3.