For the best reliability of getting the snapshot I would recommend using the “Motion snapshot ready” trigger card. Sometime the snapshot can take several seconds to transfer if Homey or the network is busy.
If you are using the camera motion then you don’t need to update the snapshot as that is done automatically. That card is meant for situations where another sensor is the trigger source. If you have both Motion enabled and use the update motion snapshot card then it can end up doing two updates which will slow things down.
As for the random trigger, I’m not sure why that would be happening. is it possible that the new firmware has changed the sensitivity of the detection?
