Photoshop :: Stretching Bitmaps All Of Sudden Feathering Parts?

For many years, I've been able to stretch bitmaps with a simple, repeating style (eg. a single block of colour, some lines etc, nothing complex) whenever needed, with no problem. This I do by first selecting the part of the simple bitmap I wish to stretch with the marquee, then using scale (apple + t) to stretch it either horizontally and vertically. End result, the bitmap is extended, looks solid,sharp, all is good.

Lately, and consistently, i've discovered a fresh hell in that this no longer works. When I repeat the process above, all looks good, marquee, stretch... then the second I commit the transform, part of the bitmap beginning where I stretched from gains a feathered/opacity effect. For years, solid, now it's gaining a feathered section highlighting where I made the transform.

Something must have changed (it's not the version of PS) in my settings, which i've hit or shortcut-ed by accident.

The only way I can "fix" the process is by ensuring I only marquee the last 1 pixel strip (either horizontally or vertically) and stretching just that. This works fine, like it used to, but is time consuming and driving me insane. I cannot fathom why it's creating this weird feathered effect when I'm stretching a solid bitmap, when I know it didn't used to.