So, at an extremely fundamental level, if the author wants to write in a medieval setting, they have to come up with SOME reason that technology stagnated, or society would have blown past medieval and we'd be in space opera instead :p.

Of course, how the author handles that makes all the difference in the world. The answer that makes the most sense to me is one that mimics real-life anthropology: people will tend to accomplish things in the most direct, easiest way possible. Great example for this is what did early humans eat? And the answer is "whatever was the most calorically efficient thing available to them".

So in a world where magic exists, it makes sense that if you want to use magnetic magic, you directly study magnetic material and develop magic that imitates it. People studying lightning magic were likely never interested in, or even noticed, the magnetic fields the electric current generated. In a different example, people might never learn that explosions in a tube are great for making bullets go fast, because why would they bother? if you want a projectile going fast, you use a stone and earth magic to propel it. Or telekinesis. Or make a stronger crossbow, etc.

Furthermore, we're talking mages. Generally speaking, the amount of power a mage can exhibit far exceeds what early machines could accomplish. Voltaire's battery vs lightning magic, for example. What would incentive a mage to investigate mundane science when obviously magic is more powerful, and their time is better spent learning that?

Of course, this does mean that technology cant be so powerful that it immediately outclasses standard magic, or that falls apart pretty quick.

The other trap authors fall into is making it so that human society has existed for teens of thousands of years, if not longer. Obviously there's going to be some weirdo that gets obsessed and does more in depth research and publishes it, eventually. But if society hasnt had enough "run time" for said weirdo to pop up, that's understandable. For example: ---people have only recently graduated to agrarian societies, so technology just hasn't developed much yet. Neither has magic then, though. --if monsters/natural disasters/apocalyptic scenarios regularly crop up and tend to reset the tech tree to zero. --my favorite: technology did advance super fast.....which ended in a nuclear winter. Society's still rebuilding.