external thermostat (what are you using)

I've never had a problem with my A419s, as long as their settings were correct.

It sounds like you want the fridge to turn on at 41F and turn off at 37F. If so, make sure the internal jumper is in the cooling/cut-OUT position. Then set SP to 37 and the DIF to 4.

From your description, it sounds like your jumper is set to cooling/cut-IN. If so, and if you want to get the desired effect without opening the unit and changing the jumper, just set your SP to 41 and leave your DIF at 4. It should behave the same way as what I described above, but you'd have to make sure that if you change your mind about your temperature target, you'd need to always add 4 degrees to the setpoint.

 

A strand of Christmas lights in the fermentation chamber with a hoodie on the fermentor.

 

I just don't remember this happening before. For example, if I set fermentation temperature to 65 with a +/- 1 degree differential, the unit kicked on when it read 67 and shut off when it read 63.

 

Differentials (DIF) are not plus and minus at the same time. It's either/or, depending on jumper settings. Your memory is failing you or you have a defective unit. What you described before, and what you're describing now, are two different behaviors, and neither one is consistent with a properly set-up A490. And they can't both be (erroneously) happening with the same unit, unless one defect replaced another. It will cost you nothing (except 5 minutes of your time) to check and set the jumper.