I did some testing, and it seems that doors DO work right, as long as they are not made to spawn at the same time, so negke's original suggestion was correct. So it looks like some other dm modification of mine broke my map causing some func_doors/plats to float in random places (they can't be touched and are visible at some points and then disappear at some other points; maybe the place-dependant visibility has something to do with vis, which dictates when to render them). Guess I gotta save and test my map more often, when doing the dm modifications next time.
My
test map has four doors, or actually seven doors, where three of the doors have two doors overlapping each other. One of the doors in such a door set is a regular func_door with no triggers, while the other one is a door with the targetname "killable door".
The first door from the left is a door, which spawns both of the doors regardless of skill level or deathmatch flags. That door glitches out always when I play the map: When I press a button that killtargets the killable door, it also eats the other half of the regular triggerless door (a double door, which btw should be opening when I get close enough, but in this case won't budge).
The second door set is built in a way, that has both of the doors spawn only in easy skill level. In all other modes only one of the doors will spawn. That door works, as long as you don't play the map on easy, i.e. when only one of the doors spawn.
The third door set is a variation, where the other door spawns only in deathmatch and the other only in single player. No problems there, since there never is a situation where both of the doors spawn.
The fourth door is kind of unrelated to the whole dilemma. It's just a test on an alternative method on how to have the doors behave differently depending on skill level or deathmatch flag. I just put four trigger_multiples in front of the door, each opening the door in their respective play modes.
One final note: While writing this post and after already having uploaded the test map, I decided to fiddle around with the "don't link" flags, after remembering Qmaster's suggestion. Maybe the door linking causes the killable door to kill some of the triggerless door. I tried adding the "don't link" flag for both doors, which sure enough prevented the killtarget from killing any part of the triggerless door, but the triggerless door still refused to open upon approaching it. Only now did I discover that the "don't link" flag disables the func_door's ability to open automatically without a trigger. I was able to solve the problem though, by giving the door a triggername "regular door" and then putting a trigger_multiple in front of it.
So thank you, Qmaster! :) It seems that door linking was the root cause of this weird "killtarget door affecting other doors" behavior! Now only if knew just what it was that caused my doors and platforms glitch out in my main map... T_T