Have the same experience - the small/cheap digital ones really suck big time. In the growroom with direct light exposure, radiant heat from HPS it's even worse.
The analog ones are better, though still not 100% precise ofc, but at least, can be easily calibrated to show the same return value in a given common situation
If been their many times moat themors are shit even some expensice ones
I now buy 3 of the same makes off Amazon the good old white ones like 9 pound I buy off 2-3 sellers and test then together and if 3 off the same seller read the same i use them lol and esure the rest through Amazon free.
I find my fan speed to stroller is always pretty close to spot on so try going off the fan and see what themors match the fan controller
If 3 off Same seller are out then they probbely junk
Iv also bough 6 I inkirds temps controller they are all out all ready diffrent