Tv and Av mode; changes

As I understand it when you set the shutter speed in Tv mode it should not change; under any conditions.

But I've noticed changing shutter speeds in Tv mode; and I can't understand why that would be happening.

For example, I had my shutter to 60, but when the light got too low for 60, I noticed a lesser shutter speed flash back when I squeezed the shutter release.

Are you sure it isn't flashing your aperture letting you know it isn't wide enough? That is how mine behaves in TV.

Sounds like you have safety shift enabled. It is designed to do just that, when the aperture won't open wide enough. Just turn it off.

Yep, you've got Safety Shift on.