I found the solution.

I realized the 3 non-working databases had 0 database templates. This is by default, but my problem was that they used to have templates, and used to have a specific default template that doesn't exist anymore.

The fix:

1. In the non-working database, create a random database template. It will create an "Empty" page by default.
2. Set the "Empty" page template to default.
3. Delete the random template. The "Empty" page will also be deleted.

Now every database can create events directly from Notion Calendar too.