Quite a few units will increase the set point once the external ambient air temperature reaches a certain point, 158s are meant to do it to avoid over stressing the components, usually approx 27c it will start increasing the set pint from 21c, so when it's 30c external it should be 24c internal. To have it try to reach 21c come what may just leads to failures, and as 158s don't have the most powerful system that's why poorly maintained examples have non working A/C..... That and Guards who don't know what they're doing and opening windows or vestibule doorways with the system still on and forcing the system beyond design parameters.
I've spent the past 5 years really pushing A/C traction knowledge and it really infuriates me when I see a set with perfectly good A/C come in with opening windows having been 'sabotaged' by a member of staff.