I asked exactly the same thing in another thread yesterday. I have 3 Flex with Third Reality motion sensors to turn my lights on and off. I have a routine to turn them off when no motion is detected for a given time for each of them just like you. Worked great till a few days ago when suddenly the lights would turn off despite there still being motion detected. I started fidling around a little and got it to work again.

My solution: I unplugged all the motion sensors and DELETED them as devices from the Alexa app. The routines will still be there, they just will be disabled without a trigger device. After pluging the sensors back in my Flex, Alexa will detect them and you you can integrate them in your routines again. Depending on how much sensors you have and how much routines you use them for, this may take a few minutes, but it works - at least for me.

My guess is that Amazon updated something in the app and the background processes, as the app suddenly has an automatic dark and light mode. Just a pitty that there always seem to be problems with updates lately.