Generally it should. If it's not - it may be the wrong type of flux. Without knowing what flux you're talking about it's not possible to give you a definitive answer.  let me ask a stupid question and please don't assume I'm thinking you're not that smart - but after adding flux - you ARE re-heating the joint; right? [end edit]Check my point 1. If I add fresh new flux, it doesn't go from dull to shiny. So I don't think the answer is that simple.
This is the #1 reason for using a flux core solder. Adding additional flux won't hurt. Again, as long as it is the right type.oxidizing the surface of solder.
Even that is slow. But depending on the conditions, materials and consumables - - - at a very least it shouldn't take more than one second for a typical hobby joint.I make 7 joints in 7 seconds.
I know you said this was an example of trying to make a joint look dull, but this can also be the result of soldering on a board with large ground and power planes. The board draws so much heat away from the solder joint it's impossible to solder without pre-heating the board. Since you're talking about forcing the issue - I don't think you're working on ground or power planes.something is wrong it it takes 7-seconds to melt enough solder to make your joint.
I've seen this time and time again where solder, before it freezes, it's moved, resulting in a grainy and dull appearance.a cold joint isn‘t about the temperature used to make it. Instead it is a joint that was mechanically disturbed while the solder was in its plastic range.
Another reason why a joint could look poor is if the lead to hole (assuming through hole joinery) is the wrong size hole for the lead. Putting a thin 0.045" lead into a 0.125" hole results in a lot of solder needed for the joint. More solder means more time the process will take. It'll also take longer to cool, allowing time for oxidation.
Solder in its solid form is literally frozen solder. Frozen doesn't mean cold to the touch such as ice or snow. Stones are frozen even in the Sahara. Lava is stone in liquid form. Crust is that point where the surface has cooled sufficiently to begin solidifying. I don't know the temperature range of lava so I can't speak with any authority on the subject. And we're not discussing lava. Nevertheless, solder has three states; solid (frozen), plastic (becoming liquid but not yet flowing) and liquid. Not certain but I think there may be a solder chemistry that literally has no plastic state. But I can't think of any examples of that being a certainty. It's just something I believe I've heard.
After 7 seconds oxidation can be so overwhelming that ordinary fluxes can not restore its shiny appearance. While some may recommend removing and applying new, I've found applying just a very tiny amount of new solder (FLUX CORE) will restore a shiny appearance.
Other things that can bother the appearance of the surface of the solder can be outgassing which is caused by a plated through hole (PTH) who's wall may have glass inclusions, thus allowing gasses from the heated PCB to blow out through the hole. I've seen PTH's with glass inclusions that got copper plated. When the lead is pushed through the PTH the glass breaks - opening a pathway for outgassing. Outgassing can also be caused by other contaminants within the PTH itself or on the lead.
As for what it means chemically - I'm not a chemist and can't answer that part. Summary: Wrong flux, insufficient flux, oxidation, plastic (disturbed) solder, contamination in the hole, lead or even on the solder tip (though the last is rare). One other possibility is that the solder itself may be old. Working in the defense electronics arena solder had an expiration date at which point I'd end up with tons of free solder that otherwise would be tossed into the landfill. Though expired, it's still very useful in hobby electronics. I'd assume an expiration date is the same for medical device manufacturers and other arenas where life/mission critical are serious concern.