Hi @ma_wu,
The lowest current for the LM4040 -5V to work is:
IRMIN Minimum Operating Current 54 uA.
Ref: https://www.ti.com/lit/ds/symlink/lm4040-n.pdf?ts=1634661159426&ref_url=https%253A%252F%252Fwww.google.com%252F page: 25
If you have it for 5V and you are feeding it with 5V, theoretically the current will be 0 uA and the LM4040 -5V is not working.
As it is not working, any measurement of values becomes invalid.
Try feeding it with some voltage like 5.3V or 5.4V.
RV mineirin