Steps to reproduce the issue

• login to administrator
• edit an existing item with a repeatable subform (the subform should have more then 1 entry already)
• delete the first entry of the repeatable
• add a new entry to the repeatable
• save the item

Expected result

at the function JModelAdmin::save($data)

all entries of the subform should be in the $data object

$data['subform']['1']
$data['subform']['2']

example code in Editform:

<div class="subform-repeatable-group subform-repeatable-group-sr-0" data-base-name="eventdates" data-group="eventdates1">
<div class="subform-repeatable-group subform-repeatable-group-sr-0" data-base-name="eventdates" data-group="eventdates2" data-new="true">

Actual result

at the function JModelAdmin::save($data)

the entry you added get the same id like the existing entry. So the new one will override the existing one.

$data['subform']['1']

example code in Editform:

<div class="subform-repeatable-group subform-repeatable-group-sr-0" data-base-name="eventdates" data-group="eventdates1">
<div class="subform-repeatable-group subform-repeatable-group-sr-0" data-base-name="eventdates" data-group="eventdates1" data-new="true">

System information (as much as possible)

Joomla Version 3.9.X (tested on 3.9.12 and 3.9.14)

Additional comments