Sources in additional-repositories.list are not shown

[dotnetCarpenter]

Repositories are not shown from additional-repositories.list file, which makes it difficult to pin point errors such as broken repositories. See https://forums.linuxmint.com/viewtopic.php?f=47&t=305954 for an example.

cat additional-repositories.list

deb https://launchpad.net/~mc3man/+archive/ubuntu/bionic-media bionic main

System info

Linux Mint 19.2 Tina Cinnamon x86_64 5.3.0-23-generic

System:    Host: dotnet-ThinkPad-S3-S440 Kernel: 5.3.0-23-generic x86_64 bits: 64 compiler: gcc v: 7.4.0 
           Desktop: Cinnamon 4.2.4 Distro: Linux Mint 19.2 Tina base: Ubuntu 18.04 bionic 
Machine:   Type: Laptop System: LENOVO product: 20AYA01100 v: ThinkPad S3-S440 serial: <filter> 
           Mobo: LENOVO model: 20AYA01100 v: 0B98405 Std serial: <filter> UEFI: LENOVO v: J3ET56WW (1.56 ) date: 11/29/2013 
Battery:   ID-1: BAT0 charge: 32.6 Wh condition: 33.0/46.0 Wh (72%) model: SMP 45N1139 status: Charging 
CPU:       Topology: Dual Core model: Intel Core i5-4200U bits: 64 type: MT MCP arch: Haswell rev: 1 L2 cache: 3072 KiB 
           flags: lm nx pae sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx bogomips: 18356 
           Speed: 881 MHz min/max: 800/2600 MHz Core speeds (MHz): 1: 798 2: 798 3: 798 4: 798 
Graphics:  Device-1: Intel Haswell-ULT Integrated Graphics vendor: Lenovo driver: i915 v: kernel bus ID: 00:02.0 
           Display: x11 server: X.Org 1.19.6 driver: modesetting unloaded: fbdev,vesa resolution: 1366x768~60Hz 
           OpenGL: renderer: Mesa DRI Intel Haswell Mobile v: 4.5 Mesa 19.0.8 direct render: Yes 
Audio:     Device-1: Intel Haswell-ULT HD Audio vendor: Lenovo driver: snd_hda_intel v: kernel bus ID: 00:03.0 
           Device-2: Intel 8 Series HD Audio vendor: Lenovo driver: snd_hda_intel v: kernel bus ID: 00:1b.0 
           Sound Server: ALSA v: k5.3.0-23-generic 
Network:   Device-1: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet vendor: Lenovo driver: r8169 v: kernel port: 3000 
           bus ID: 03:00.0 
           IF: enp3s0 state: down mac: <filter> 
           Device-2: Intel Wireless 7260 driver: iwlwifi v: kernel port: 3000 bus ID: 04:00.0 
           IF: wlp4s0 state: up mac: <filter> 
Drives:    Local Storage: total: 465.76 GiB used: 108.01 GiB (23.2%) 
           ID-1: /dev/sda vendor: HGST (Hitachi) model: HTS725050A7E630 size: 465.76 GiB 
Partition: ID-1: / size: 94.28 GiB used: 53.99 GiB (57.3%) fs: btrfs dev: /dev/sda5 
           ID-2: /home size: 94.28 GiB used: 53.99 GiB (57.3%) fs: btrfs dev: /dev/sda5 
Sensors:   System Temperatures: cpu: 46.0 C mobo: N/A 
           Fan Speeds (RPM): cpu: 0 
Info:      Processes: 224 Uptime: 1h 26m Memory: 3.76 GiB used: 2.10 GiB (55.7%) Init: systemd runlevel: 5 Compilers: 
           gcc: 7.4.0 Shell: bash v: 4.4.20 inxi: 3.0.32 

[xenopeek]

The problem is you put the link to the PPA website in your repository configuration, not the link to the PPA repository. That is, if you click on the link https://launchpad.net/~mc3man/+archive/ubuntu/bionic-media you're taken to the PPA website. It has instructions there for adding the PPA to your system or in Software Sources you can on the PPA tab just click Add and input "ppa:mc3man/bionic-media".

This is what you put in your repository configuration:
deb https://launchpad.net/~mc3man/+archive/ubuntu/bionic-media bionic main
This is what is what it should actually be for the PPA:
deb http://ppa.launchpad.net/mc3man/bionic-media/ubuntu bionic main

As such mintsources doesn't see your line as a PPA and instead of showing it on the PPA tab (because your line isn't a PPA repository), shows it on the Additional Repositories tab: