Rule to enforce switch-on brightness level Fibaro RGBW

[rossko57]

@sjstrobl

When I turn OFF the light from a higher value (such as 80%), then the light goes out slowly

That’s a feature of your device.

@sjstrobl

for some reasing during this period the rule is being triggered again

Because your device reports it status periodically.
You openHAB Item gets set to 0 “prematurely” because you have got autoupdate enabled (by default). But then the real dimmer comes along and says “I’m not 0 (yet)”.
See “jitter” here -

Autoupdate - a primer

I’ve found myself having to investigate and/or explain some of the features of autoupdate over time, and thought an in-depth view would be helpful. These notes apply to openHAB version 2.5 Overview Autoupdate is primarily a “speed up” feature for openHAB Items. Mostly we can ignore it, but sometimes you may get an unwanted or unexpected effect, or you may wish to exercise more control. To fully understand autoupdate, we must understand the difference between command and state of Items. If w…

If you look at your events.log, you will see the sequence.

Disabling autoupdate on your Item is a quick fix for commands from openHAB side - you didn’t say but I’m guessing the rule works fine if you control from outside of openHAB.

Otherwise, you’ll have to do something more elaborate, probably involving a brief timed lockout.