Surprise, surprise!
It's working the other way round:
Q15 is (always) off, separating the lower MUX part from the signal path.
AZ is obviously realized with Q23, Q24, as there is no difference with AZERO ON.
pin5 of U8A is exactly zero Volt, like all comparators U8A-D and U7A-C.
Then this modus operandi explains, why 100V and DCI are working correctly.
Q21 is always on, obviously its Gate diode is conducting in forward direction. U7D, pin 11 has 5.0V signal. The T-sensor U21 is NOT powered on its pin1, obviously only switched on briefly for TEMP?
Therefore R8 (13.5k) pulls the MUX bus towards zero, plus this odd level at, and current through the Gate of Q21.
Please check accordingly with a 470 Ohm resistor between U9 pin 12, and GND on pin 8, whether the shift register is faulty, or U8A.
I can't imagine, what happens, if U8A draws excessive current out of output Q6 (pin12), whether this could also influence the level of its Q0, or what might be the failure mode of this shift register.
But I'm pretty sure, that either of both ICs is causing the trouble, very probably U9 U8 is it.
Frank
PS: Yes, in both DCI and 100V range, Q21 is turned off, i.e. 0V at U7D, Q15 is turned on, +5V at 8D, and either U7A / Q18, or U8C / Q16 is turned on, 5V at either comparator input.
So you can also check these voltages, whether U9 delivers these correctly.