Authentication keys not shown with "signed-by" option, repository not shown when using *.sources file

[State-Of-Joshing-Gentle-Peevishness]

Describe the bug
When saving authentication keys of additional repositories to a location outside the apt-key area (using signed-by=<path-to-keyring.gpg> in the corresponding sources entry), the authentication keys are not shown under software sources > authentication keys.
In addition, when the sources entry is a *.sources file, the repository is not shown under software sources > additional repositories

To Reproduce
Steps to reproduce the behavior:

1. Using QGIS repository as an example: https://www.qgis.org/en/site/forusers/alldownloads.html#debian-ubuntu
2. If you follow the instructions, it will generate/use the folder /etc/apt/keyrings/ to safe the signing key and create the sources-file /etc/apt/sources.list.d/qgis.sources with Signed-By: /etc/apt/keyrings/qgis-archive-keyring.gpg
3. After following the instructions, the update manager doesn't show the signing key under software sources > authentication keys and doesn't show the repository under software sources > additional repositories
4. The signing key is also not shown if it's path is referenced in a *.list file with the singed-by option

Expected behavior
The signing key and the repository should be shown.

Distribution:

• [x ] Linux Mint

Software version:
5.8.8

Additional context
As far as I understand is apt-key deprecated for security reasons and now signing keys must not be placed anymore in /etc/apt/trusted.gpg.d or loaded by apt-key add, but be referenced with "signed-by" option. Debian wiki reference: https://wiki.debian.org/DebianRepository/UseThirdParty

[github-actions]

This issue has been automatically marked as stale because it has not had
any activity in over 2 years.

If this issue is still relevant:

• Please test on a current version of this package.
• Add a comment with your findings

If no activity occurs in the next 30 days, this issue will be
automatically closed. Issues can be reopened if needed.

[SineSwiper]

This bug is likely still an issue, due to #1036.