I've always played that Combat Armor is not a space suit, simply put. It can operate as one in a pinch, but frankly, the manufacturer makes no guarantees. It is sealed because it's supposed to be usable in a CBNW battlefield and has a certain amount of environmental support and sealing (to keep out nerve gasses or biological agents), comfort related items like heating elements, limited radiation protection, and so on. However, it's not designed to be a spacesuit and was intended to be used in an atmosphere.
"Great, so how is that different from an armored vacc suit, then?"
1. Combat Armor would have a lot of trouble radiating excess heat in an orbital situation (which is a far bigger problem than the so-called "cold" of space) - CA would be designed with the idea of shrouding thermal signature and assuming there would be air to bleed heat off into. Strong, unfiltered sunlight hitting (as could be found in the life zones of star systems) the suit for any extended period of time (say, anything more than 30 mins) would cause CA problems with heat dispersal - where heat would be expected to be pretty easily radiated using fans or some sort of high-tech capillary action to pump air around.
2. Vaccum Welding might be an issue - since CA is meant to be used in an atmosphere, parts like gaskets and similar parts would not have stuff like vaccum-tolerant lubricants. In addition, CA might have various hooks or clips designed to hang things like grenades, ammo pouches, combat knives, and so on the outside in ways that assume gravity and atmosphere.
3. CA might also be built with the assumption of atmospheric pressure pushing against the armor as opposed to be being constructed to hold pressure in. Places like joints and so on might be placed under undue stress and perhaps eventually "balloon" and blow-out in a vacuum under extended conditions.
4. CA would depend more on filtering/scrubbing surrounding air supplies as opposed to carrying its own. In addition, CA would be fitted with positive air flow systems (ie; if the armor is punctured, air would continually squeeze out of the hole to keep potentially harmful stuff outside) and depend on user intervention to seal the hole. Spacesuits above a certain TL are self-sealing and much more concerned about keeping the atmosphere of the suit inside of it.
5. Radiation shielding might be an issue as well - CA would probably be designed more to keep contaminated radioactive matter away from a soldier (mostly dust/dirt), and otherwise the soldier would be expected to stay away from the nasty stuff (not staying in irradiated areas long) and going through proper decontamination afterwards. There would be some shielding, but nowhere near the amount of a "real" vacc suit. Trying to use CA as a vacc suit would mean exposing yourself directly to various harmful cosmic rays that are going to hit regardless.
Would any of these factors prevent CA being used as a spacesuit for short periods of time for a few times of the lifetime of the suit? No.
Will CA in good repair prevent you from dying a horrible death from vaccuum should your ship lose life support? Yes, it will.
Would it be a good idea to use frequently or as a stand-in for a spacesuit? No.
Would the manufacturer recommend it? No.