This is highly speculative, though it happens often enough to where I have a theory.
There are muscles that control the lens for flattening as well. If you have a less than ideal balance of close-up vs. distance, those muscles may have to work to flatten the lens for distance, rather than just the ciliary relaxing from close-up focus.
It’s a fairly decent theory though of course we have no way to go actually look in the eye.
What tends to help with this is gradually reducing some more close-up and adding more distance. Nothing extreme or rapid changes though, just to where you find eventually the opposite taking place – distance vision being relaxing rather than strenuous. Sometimes it’s nothing significant, just maybe a break more often or finding something else to add into the routine that excludes screens.