There are cases where a players backpack will go missing when drop on death. Yes my settings is Drop on Death TRUE.

It doesn't happen everytime but it does happen quite frequently. Will this be fix?

Yes same here.

i've been experiencing the same but mine drop through the terrain, they don't actually go missing.  Still an issue though.

Mine has been happening on cargo and oil rig.

This should hopefully be fixed with update 3.0.5 and later. Please get back to me if the issue is still persistent.