Forms can't be attached to posts

While the available blocks for posts/articles include attaching a form, this feature needs to be fixed. The form appears to be attached but disappears on a page reload.

Expected:
1. Open a post from `/admin/posts`
2. Click on `Add new block` and select `Form`
3. Click `Attach`, select any form you'd like, and save by clicking `+ Attach`
4. Go to the top of the page and click `Save`
5. When refreshing, the form still appears attached to the article

Actual:
1. Open a post from `/admin/posts`
2. Click on `Add new block` and select `Form`
3. Click `Attach`, select any form you'd like, and save by clicking `+ Attach`
4. Go to the top of the page and click `Save`
5. When refreshing, the form **no longer appears attached** to the article

Screen recording showing the issue:

https://github.com/code4romania/website-factory/assets/2820084/412f0909-a494-42e5-ab3c-53a888d8db5f