Medigun beam disappears when spectating in first person

Description

https://github.com/ValveSoftware/Source-1-Games/issues/935

Tested in latest tc2 build

I strongly suspect that this is code related, as there are other instances where code hides particles in first person when spectating, for example

bool C_TFPlayer::CanLightCigarette( void )
....

    // Don't light if we're spectating in first person mode.
    if ( (pLocalPlayer->GetObserverMode() == OBS_MODE_IN_EYE) && (pLocalPlayer->GetObserverTarget() == this) )
        return false;

...

The only thing that may be related is this alledged undocumented changed from the Oct 6, 2010 patch

"Attached weapon particle effects removed from viewmodel (still present in 3rd person)"

However, I couldn't find any info about it.

Checklist

[x] None of the open or closed issues document this problem.
[x] This is a bug, not a new idea.
[x] This is objectively a bug, and not something that people commonly disagree on.
[x] This issue occurs on the latest Team Comtress release, or the master branch.
[x] This issue occurs on the latest vanilla TF2 update.
[ ] This bug is in the code of the game, not in the textures, sounds, or other resources.
[x] This issue is listed on the Source 1 official bugtracker.
[x] This issue occurs with DirectX level 100.
[x] This issue occurs when using no custom content.
[x] This issue occurs when using start_tf2.bat (or start_server.bat) with no custom launch options.
[x] This issue occurs when using a listen server (using the map command ingame).
[x] This issue occurs when using a dedicated server.
[ ] This bug is in the internal procedures or tooling (e.g. the build process).

Reproduction

Spawn a Medic bot
Spectate it

Screenshots

https://github.com/mastercomfig/tf2-patches/assets/63692647/5e0044c9-133f-4ed9-947c-a7b56196c3fa

Setup Details

Client Settings

Server or client: Both
Operating system: Windows 10
Team Comtress version: 2.0.2
Custom content: No
Launch options: Default
Driver Version
DirectX level: 95
Hardware Specs
CPU model: AMD Ryzen 5 3600
GPU model: NVIDIA GTX 1650 Super
RAM amount: 16
TF2 install location: SSD
TC2 install location: SSD
TC2 installation method: Basic

mastercomfig / tf2-patches

Medigun beam disappears when spectating in first person #661