This issue came up a few time in the dod forums.

This post... Invisible crosshair while prone and weird ladder physics (linux)

When I am prone, I can't see my weapon nor can I see my crosshair. In addition to this, ladders not just in this game but also in TFC are funky. I notice that I seem to slide up and leftwards when moving forward on a ladder, sometimes its like I am flying off no matter what I try. I was looking through the forums and someone had both of the same problems as me but with no resolution. I am running the native linux version of half life. I have tried both the anniversary version and the legacy version while running day of defeat. I am assuming it is some issue involving the linux base of half life with the mod itself.

So, I don't think Linux or Windows matter here.There is a solution posted on reddit for the same issue. No Crosshair when prone/Sniper crosshairs messed up after HL HD patch

Hi Guys n Gals,Ever since the half life HD patch I have no crosshairs when I go prone which makes going machine gun a little hard, additionally the sniper zoom no longer fits my screen. I was told a fix a while ago on a server however I can't for the life of me remember what it was! I know it was a simple one.Can anyone help?

 

Look at this solution, it looks like Joysticks or gamepads are the core issue...

Right click your game in steam library, go to properties, in the "controller" menu disable the steam input for override for dod.

That should fix the no xhair while prone and the ladder glitch. If that doesn't solve both issues, unplug any controllers, steering wheels, or peripherals that aren't mouse and keyboard before you launch dod.