this has been an annoiance for quite some time. I always had the feeling that updating rules for kids don't work on the kids devices.
Now I saw it is very easy to reproduce with the vacation mode. When I set it, the setting automatically disables again after approx. 10 seconds.
This happens at least on two devices: Moto Z on Android 8.1 and Xiaomi Mi A1 on Android 8.1
See attached video.
Can you get this fixed soon, as my daughter is on trip right now with an almost unusable device.