The sensors report the number in % which is then converted to lux. 100% light equates to 80 lux, so you’ll never see anything above 80 lux. (again these approximations based on tests and manufacturer specifications and not a 100% accurate as the device is reporting the light level in %'s and not lux).
You can change the settings from the preferences menu to choose between the native % and translated lux and see which metric works better for you. There is also a known issue with some devices (firmware) where when it does totally dark it reports a 100% level. Sometimes a reset helps or you could request a replacement device.
100% mean the light sensor is saturated so it’s best to use them indoors as a proxy for external lighting conditions. E.g. mount them above a window facing away so that the level of lights reaching them are reduced (reflected or dispersed) and that tends to be more accurate than placing them in direct light or very bright conditions.
Also check your Light Sensitivity settings (too sensitive may reduce battery life, not so sensitive may take more time to update, for testing purposes you can make all sensors highly sensitive and compare the readings).
As for the getting stuck, try this, the next time it’s stuck, press the button in the pin hole at the bottom of the sensor (this forces the sensor to send a refresh). If you see that the value is now reported correctly then this indicates that you sensor is too far away from the nearest Z-Wave repeater and likely the last update was lost in the mesh somewhere. Try adding a repeater or two between the sensors to make the mesh stronger and resilient.
If the value doesn’t update then the sensor may be “stuck”, usually a simple reset works to fix this. See this post on how to reset the sensor
FACTORY RESET: Remove the cover and press and release the Z-Wave button with a pin (tiny hole at the bottom of of the sensor) 10 times within 10 seconds.
Usually doing a reset often helps with inconsistent readings