IMPP property incorrectly formatted/read in vCard 4 breaking RFC

ISSUE

Nextcloud creates/syncs/exports the following (vCard 4) for IMPP instant messaging fields:

E.g. Contact with a Skype IM address of https://skype.com/sfe

GROUP3.IMPP;TYPE=x-skype:https://skype.com/sfe
GROUP3.X-ABLABEL:Skype

CAUSE

Nextcloud is applying the vCard 3 (extension) standards, and not the correct vCard 4 standard.

vCard 3 that Nextcloud is using for vCard 4 also:
https://tools.ietf.org/html/rfc4770
RFC example: IMPP;TYPE=personal,pref:im:alice@example.com

vCard 4 that Nextcloud should be using for vCard 4 versions:
https://tools.ietf.org/html/rfc6350#page-37
IMPP;PREF=1:xmpp:alice@example.com

RESULT

This does not accord with the RFC spec, and thus is incorrectly translated by other systems (such as Android via DavX) and appears wrongly: in this case: Label: https Value: //skype.com/sfe

DavX/Android correctly create the following value of the same field:
IMPP:Skype:https://skype.com/sfe

RFC ( https://tools.ietf.org/html/rfc6350#page-36 )
Specifies the following example: IMPP;PREF=1:xmpp:alice@example.com

Note that

1. PREF is optional
2. TYPE is not provided for this property
3. A URI can be provided, but note that most modern systems recognise common IMPP parameters (Facebook (messenger), Instagram, Twitter, Skype) with just the username.

RESOLUTION

Read and write the correct vCard 4 specs for IM fields as above.

No file or test is named, so first locate the IMPP read/write entry point in the contacts code and compare its behavior with RFC 6350 pages 36–37, linked in the issue. Done means vCard 4 IMPP fields are read and written using the RFC 6350 parameter format rather than the vCard 3 TYPE format.