I found out that in the Base->Aircraft->Assign soldiers screen, the soldiers' health bar may go missing. After fiddling with the code, I found out that it was due to some really strange code in 0ufos.pk3/ufos/menu_team.ufo:
string hp_val
{
if "mn_show_employee == 1"
pos "975 360" string "*cvar mn_thp" align ur
}
bar hp_bar
{
if "mn_show_employee == 1"
pos "716 383" current "*cvar mn_vhp" size "245 6" color "0 .8 0 1" max "*cvar mn_vhpmax" align uc
}
I searched for mn_show_employee in the C code and found it only in the section that deals with hiring staff, so I suppose the condition in the above code is wrong. After commenting out the condition, the health bar is displaying correctly. Also with the condition above in place, I verified that setting the Base->Hire staff screen to anything other than Soldiers will indeed disable the health bar.