[Feature Request] Support RELTYPE=CHILD for RELATED property

Hey,
I am writing a ToDo-List app for Windows (UWP) and I want to add a Subtask to an existing ToDo and for that I use the RELATED-TO property. I encounter a problem with the following VTODOs,
but i really need the RELATED-TO;RELTYPE=CHILD on the parent to determine its children...

Steps to reproduce

Put these to VTODOs in your calendar:

Supposed parent

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Sabre//Sabre VObject 4.1.6//EN
BEGIN:VTODO
SUMMARY:LIST1153
DTSTAMP:20200401T095356Z
UID:2fa55d38-006d-4010-aa3a-dc4f392cbccd
LAST-MODIFIED:20200401T095356Z
RELATED-TO;RELTYPE=CHILD:255ce6ca-fbae-4263-89c5-5743f8456b22
END:VTODO
END:VCALENDAR

Supposed child

BEGIN:VCALENDAR
VERSION:2.0
PRODID:Nextlist/Win
BEGIN:VTODO
SUMMARY:ELEM1326
DTSTAMP:20200401T112650Z
UID:255ce6ca-fbae-4263-89c5-5743f8456b22
LAST-MODIFIED:20200401T112650Z
END:VTODO
END:VCALENDAR

Expected behaviour

Correct subordination (PARENT-CHILD).

Actual behaviour

In the UI the parent is shown as subordinate although it's defined as superior in the code...

If I set RELATED-TO;RELTYPE=PARENT to the child VTODO and no property to the parent, the UI shows the subordination of the child correctly.

Nextcloud version: (see Nextcloud admin page)
18.0.2
Tasks version: (see Nextcloud apps page)
0.12.1

Start by reproducing the behavior with the supplied parent and child VTODOs, then trace how RELATED-TO relationships are interpreted for display in the UI. Done means RELTYPE=CHILD on the parent produces the same parent-child hierarchy as RELTYPE=PARENT on the child.