For the first several months I seemed to have a pretty static set of equipment. Occasionally a laser rifle might go missing (or maybe it was just still equipped to a guy in the med bay), but mostly my gear didn't disappear. Around month 4 or 5 on veteran I was not only getting slaughtered by the up-armored aliens, but I noticed my guns and armors disappearing and my workshops couldn't produce them as fast as I was bleeding them.
Is this from those plasma explosive weapons the aliens are using? I think I am losing 1 or 2 wolf armors each mission (in addition to a couple men).