I've noticed the bounding box that shows you which block your cursor is pointing at is much darker now... but that isnt the bug. I like that change. The bug is that when you have your cursor pointing at a transparent block such as ice or stained glass while there is water behind them, the bounding box becomes hard to see. It becomes even harder to see with what I described above if it is dark and or night time.
I also found that something similar happens when you have your cursor pointing at transparent blocks that are up against the nether portal while is active.
It seems as if the water and portal animated textures are rendering ontop of the block selector's outline.
I have some pictures to show what I mean.
Also I did quite abit of searching and it looked like this bug hasnt been reported yet.
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Linked to the related issue... I agree with you I can actually see the selection box now, also would like to see it stay.

I feel like the selection box becoming solid might have been a quickfix to make this less noticeable, as if it was transparent you would have an even harder time seeing it. In terms of the problem, it looks like the box isn't layered correctly. It seems like transparent textures always take priority, as with the problem when particles in front render behind, and anything requiring priority must be manually set by them.

I hope it stays too, hopefully this problem gets fixed before the official release.