So I'm not sure if this is by design, but it's not ideal UX. Plus the behavior is different from Joomla 3 so I'll assume it's a bug, if not then please close this issue.

When you use subforms -> repeatable.
Whenever you add use the + button from an existing field, you expect the new field to pop below the existing field you just clicked. Right now, regardless of where you press the + button, the new field(s) all appear on the bottom, which can result in confusion that the button isn't working if the number of fields exceed your browser height. In Joomla 3 the repeatable fields work as expected, where a new one "drops" below the existing field that you clicked "+" on.
I hope I make sense!

Steps to reproduce the issue

Create a subform in custom fields,
and use it in Article editor.

Expected result

New empty field in subform should show below the existing field if you use the + of said existing field

Actual result

Every new field is added at the bottom.

System information (as much as possible)

Joomla 4 RC1
Mysql 5.7.26
PHP 7.4.2

Additional comments