Hopefully this does not giver offense, but since you're human like the rest of us, I'll float the possibility that this observation of a sharp-edged object may have been influenced by your brain initially seeing what it thought was an OBJECT, an illusion that failed as you had more time to observe, and particularly when you realized what you were seeing, and how a fuzzy-edge was pretty likely, given that clouds were involved.
Of course, that could be wrong. The lighted patch may have looked sharper, earlier, for example if the plane was closer to the clouds, or the clouds were denser, or there was a layer of have between you and the cloud base initially, but it petered out, etc.
It would be impossible at this point to estimate even very loosely what the chances are for each of those scenarios. Which is, of course, the problem with witnesses and their memories as sources.
