This happens to me all the time when designing. I’m still surprised at times when I at first show a design on my phone. Sketch Mirror helps, but when your mind knows what is there it has a way of filling the gaps even when you see your design on a small screen with a little less contrast.
Hopefully a sketch plugin maker reads this and designs something that could scale through the different possible lighting/how contrast will appear situations. Until then you’ve inspired me to turn down the contrast on my monitor. I think I’m guilty of having it cranked up, maybe I’m not the only designer guilty of this and it is what is causing the real problem.