Imprecision
The numina warp things in their vicinity, everything somewhat differently and unpredictably. (Or maybe it's not the numina but some other agent.)
I.e. two yardsticks that had the same length today will vary by, say, 1 millimeter tomorrow.
The beams of a scale will have slightly different length tomorrow.
Your weights be 0.1% heavier or lighter tomorrow.
That kind of stuff.
Everything medieval still works - houses (wood moves anyway), weapons, armor, carts - none of this requires millimeter precision.
Cogwheels? They will work but never be efficient.
Waterwheels? That's wet wood, it will work. You can have a lot of low-precision machinery like hammer mills (leather is pretty useful for transmitting power as it is slightly elastic, steel bands won't work so well anymore).
Mass production is possible - sort of, anything that requires precision requires human labor. Weaving machines are never going to be precise enough except for pretty crude fabric, and even then it may not be worth it. Taylorism will work (that's how "manufactories" were set up, these tended to have multiple waterwheels in a nearby river, power transmitted into the working halls via leather bands).
Quantitative chemistry? Impossible: the beam length will vary so your measurements will not repeat on the next day, i.e. it will be very hard to be sure about anything. (Read up on what made Lavoisier the founder of modern chemistry.)
The inhabitants may not even be aware of the situation. The only thing that they observe will be "complicated mechanisms tend to jam on the next day".