auto stand by doesn't work

This is my second pair of vm 80 monitors and the auto standby function does not work, rca disconnected, xlr disconnected and nothing, the light does not turn red.
Is it possible that there is a defective series?
The serial numbers are the following:
First pair:
ACEC000173UC
ACEC000139UC
second pair:
ACEC000156UC
ACEC000181UC

bought all in the same store