Heartbeat: Availability of passive devices

[ppanag]

I have 2 passive devices. A temperature sensor and a PIR/motion sensor. (tuya-th02z, tuya-zp01)
It's important to know if they are working/broken in a time window of 30 minutes... (For example, for security alarm or heat controller...)
I've read the Device-Availability section of documentation and it says that passive devices can not be pinged and may send data even as low as 1/24hours. Which sounds logical, since they need to sleep to save battery power.
BUT THEN I tried this: Restarting zigbee2mqtt. And immediately after restart, both devices send data! How is that possible??
And, can we utilize this as a heartbeat? (to send health-state at smaller intervals...)