Have a weird situation:

Woke up one morning tire completely flat, took the spare out and placed it on the car and noticed it was really low, checked pressure on the spare and it was like 25lbs not the 60lbs that it should have been.
Tpms light on dash as expected, went and put air in it to 60lbs and went and repaired my tire. Drove off no tpms lights everything good.
A week or so later (and currently) every time I start the car I get a tpms warning on my dash stating check pressure on spare, it goes away after a minute or less.
Ive checked it and it was at 60lbs, I have dropped it to 32lbs thinking the tpms is programmed like the main tires, but it stayed on...
Question is if it was down to 25lbs for who knows how long (I have had the car 2 years) why did the tpms not warn me before?
I dont want to spend $70.00 on one tpms for the spare, will it go away or do i need to get a new sensor? it is just annoying.