Why not use the timer function built into the outlets app instead of routines?
These are generally more reliable.
Since Alexa and the routines are could based any disruption in internet service no mater how brief can cause unexpected results.
Also since these are wi-fi based there are lots of things that can interfere with the 2 Ghz RF signal causing the signal not to get to the outlet or a phantom signal can get misinterprited (though that is rare)
I've also had issues with routines no triggering wi-fi commands the voice notification almost always sounds but not the signal to turn on devices. I've since used the devices app and set a timer for the same time as the routine and have yet to have the device not turn on as expected.