Since there is also times when the device is unavailable, it seems communication problems might be the culprit, or the device loosing power.
If there is no cause listed in HA, it can be an event originating from the device. Either control from the device itself, or power loss, followed by the default behaviour of the device when power is restored.
But if it is communication problems, the list of possible causes is very long. The obvious: make sure your zigbee stick is on an extension cord, away from other devices. Make sure you have enough zigbee router devices. The k if your Zigbee channel overlaps with your wifi channels (be aware: they use different numbering systems). Never cut power from zigbee router devices. (Do you have other bulbs or mains powered things that get cut from power?).
The Home Assistant Cookbook has lots of troubleshooting tips.
This is a good place to start: