[Observium] Alert chekers entites 'disapearing'

Hello,

I've created alarm checker:

os equals nxos hostname match *NET*

ifType equals ethernetCsmacd ifAlias match *vPC Keep-alive link*

And after a while i noticed that some entities where disapeared. The device was reachable and etc. I did a "rebuild' and they're reappeared again. What could be the cause ? NXOS ? :)