Losing adjustments upon flattening

Yes that is why viewing the image at 100% (actual pixels) is a good idea before flattening.

 Ideally one would just be able to see the whole image at 100% view and make the adjustments, but of course with a laptop or low resolution screen, that's really not possible.

The change your seeing occurs because at views other than 100% view photoshop elements uses interplolation to show the image which can lead to inaccuracies in the image such as your seeing particularly at views below 50%.

You might consider getting a external 4k high resolution monitor.

You don't have to do the adjustments at 100% view, just check after your done to see if the image changes.

Definitely easier said than done on laptop top screens and other lower resolution monitors.

That commonly happens when the view percent of the image on screen is less than

100% (actual pixels) since photoshop elements uses interpolation to show view

percents other than 100%.

In other words, viewing an image at 100% view is the only way to see what it actually looks like.

Double click on the Zoom Tool to get 100% view or go to View>Actual Pixels before

you Flatten the image.