Mirage's eclipse changes depending on the intensity of light that you are in, however the ui does not display this variation. It will always display the maximum amount that you can get. It should display the current damage buff, or it should deal the full damage buff regardless of the intensity of light that you are in.
No buff: 162 damage https://imgur.com/ZWfWiME
Full buff: 584 https://imgur.com/QiGr3bF
Standing in partial light: 231 https://imgur.com/HShsRJ8 You can still see the 260% buff in the ui at the top right, however this this not the actual buff value.
This has been the case for a very long time and it is frustrating not knowing how much buff you are actually getting. It is also not fun having an ability that is so wildly inconsistent. I would suggest to simply make it buff to the full amount when it is in any light or dark conditions, it will make the frame much more fun to play, thanks.