Okay, if that reading was after a reasonable drive, then something obviously isn’t quite right.
The system isn’t that sensitive in general, for a rule of thumb for OUR car (i assume it’s at least similar for all others), the TPMS doesn’t sound an alarm until one tyre lost at least 3 PSI, we know that because we have a known slow puncture somewhere on a rear tyre – it’s around half a year after a fill up that the system warns us, at which point the tyre lost around 3PSI. Some might be more sensitive (depends on the kind of tyre you have, too) – but in general it isn’t a very accurate system.
And yeah, you’re right of course – if you lose that much pressure, it obviously should pop up. Since it does notify you about the rear tyre pressure loss, the system itself is working, which in high likelihood means that the front left (or right, forgot if you mentioned it) ABS sensor is broken, though that should technically lead to an ABS warning light in the dash. Since you haven’t mentioned that, i assume that’s not the case.
But it’s one of those things that can be a potential safety hazard, so don’t kick the can down the road.
Prior: SEAT Ateca Xcellence Lux 1.5 TSI DSG MY19, VW Golf GTE PHEV DSG MY23
Current: Hyundai Ioniq 6 Ultimate
Next: we'll see what's available in 2028.