Couldn't able to create fields inside ACF flexible content

I was trying to make some small updates on the wordpress website which I developed for client. But currently when I tried to add an extra field inside a flexible content using ACF Pro I found some bugs. After adding when click update, the field is added as a separate field and not inside the flexible content. It's been a long since I made changes in the website and before it was all working fine. Couldn't able to figure out what is the issue.
Wordpress version is 5.0.3 and ACF version is 5.5.0.