PertIve had this same problem for a while and have found no fix for it
It has got to be something in my comanglias config bc when I did the MM stress test I could play with no viewmodels on but I could see the tracers then. When I go back to regular TF2 with no viewmodels I cannot see my tracers.
edit - on second thought it could be related to viewmodel_fov. Since Valve restricts it to something lower than what I have set in my crosshair switcher (even though I have viewmodels off for scattergun) MM reverts it to a lower value, thus revealing the tracers when I enter MM but they go away when I join anything else. So maybe try lowering your viewmodel_fov and see if that helps? I'll test on my end when I get home
EDIT: problem solved. It appears it was an issue with broeselhud crosshair switcher. I guess in settings, inputting off_with_muzzleflash allows r_drawtracers_firstperson to work. I'd have to see exactly why, but changing the value from
alias scout_primary "medium; pink; cross; off" to alias scout_primary "medium; pink; cross; off_with_muzzleflash"
fixed it.