Why are my solder joints dull and grainy instead of shiny?

The shine is mostly cosmetic, so don't sweat it too much. A dull but properly formed joint with good wetting is a good joint. That said, your random shiny ones are a clue worth chasing.

First thing to check: what alloy is that cheap wire? If it's lead-free, matte and grainy is completely normal, that's just how lead-free looks when it solidifies. The mirror-shine joints in tutorial videos are almost always 60/40 or 63/37 leaded solder, which cools to a glossy finish. If your wire is lead-free and you're comparing yourself to leaded videos, you'll never match them no matter what you do with the iron. If it IS leaded wire and still coming out frosty, then the bargain-bin alloy is the prime suspect. Cheap no-name solder often has impurities or inconsistent flux, and contaminated solder gives that dull crystalline look. The joints that occasionally come out shiny might be getting slightly more heat or slightly cleaner flux in that spot, which is why it seems random. One other thing that causes the frosty look on otherwise good joints: the joint moving as it cools. Even a tiny wiggle during the second or two it solidifies breaks up the surface and makes it dull. Since you're using helping hands, that could explain the inconsistency too, one joint holds still, the next one trembles slightly.

My advice is just to spend a few bucks on a proper brand of leaded solder from a real supplier rather than the local shop's cheapest spool. Not because shine matters for function, but because you eliminate the variable and know exactly what you're working with. Get 0.7mm or 0.8mm 60/40 with a decent flux core and I'd bet most of your joints go glossy right away. If a few stay matte after that, they're still fine, and a multimeter check like you've been doing is honestly the real test of whether a joint is good.

Been soldering since the days when everything was leaded and a shiny joint was just how you knew you'd done it right. Those frosty joints you're getting remind me of the ones I'd get when a ceiling fan or an open window was blowing across the bench. Moving air cools the joint unevenly while it's freezing up and you get that crystalline look. Had one apprentice years back who couldn't figure out why his work looked dull and mine didn't, turned out he was hunched right over breathing on every joint. Backed off a few inches and his finish cleaned right up. Might be worth checking what's blowing around your bench before you spend a dime on new wire.

I agree with the point about cheap solder having inconsistent flux; that makes a huge difference. I once ran into a spool that was just garbage, and even when I was holding the joint perfectly still, the dull finish stayed because the surface contamination was too high. It felt like I was fighting the material itself, not my technique. I learned that sometimes getting a little extra flux applied right onto the pad and lead before the solder hits relly helps clean the surface up before it locks down.

This is making me want to go check my spool for a label, I honestly never looked. One thing I noticed on my practice board is the dull ones seem to happen more when I go back and reheat a joint to fix it, even just a little touch up. The first pass looks okay but the second one always goes frosty. Is that a normal thing or am I doing something wrong when I redo them?

Quick update - I started checking my spools, and turns out I don't know the alloy of the cheap wire I was using. But the point about reheating really made me stop and look. I found that even tiny touch-ups after the first pass always turn frosty, just like one person mentioned in the replies. I wonder if that's just the flux running out.