I ended up downgrading the FIbaro app to 2.1.16 Thursday evening (as it is the most cost efficient option to start with 
), it seems to work properly now both for motion and the tamper alarm.
The app downgrade was done in-place, so it did not require the devices to be re-added, I did power-cycle the motion sensors though. As for other devices, I only have the Fibaro Smart plugs which also still worked properly straight away.
So for those who are experiencing the same issues, it might be worth downgrading to 2.1.16, the commit I used was: https://github.com/athombv/com.fibaro/tree/fefc4e27b01242ebfaa8f076958cc9568b109f72
For installing / downgrading the app, the following how-to is available on the forum: [HOW TO] CLI install method
