Frustrated by a recurring problem with curves and sometimes color balance PS CC macOS Mojave

I am at my wit's end with a recurring PS CC problem. I am working on a one year old iMac with Mojave and  I have updated PS to newest version before and it has made no difference. When I select a part of an image, then alter curves or change colour balance, something very strange happens. In the image below, the second crop shows the selected doorway gone all different colours and all I meant to do is to make the area darker. In the third crop the rug the cat is standing on is meant to be grey as in above pics but when I apply curves, it goes all psychedelic. It's like it picks up different parts of the spread and repeats it in a random way, in different colours. I would really like to get this sorted out cos it happens 80% of time I hit curves. I know there is adjustment layers but this is the way I prefer to work. It's really slowing me down and killling my buzz.

Thanks in advance!

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