Trying unsuccessfully to mirror a selection

I see two things you probably need to change, to get what you want:

1. As the others noticed, the selection shown in the GIF animation is not the wing, but everything except the wing. The evidence for this is the Select Inverse step in the History panel, and the second selection marquee at the canvas border. The resulting inverted selection is the canvas minus the rectangle containing the wing. But if your intention was to flip the wing, the selection needs to be inverted back (un-inverted) so that it’s the wing again.

2. It sounds like your goal is to duplicate the wing. If so, there is one command you need to change. Even if the wing selection was correct, the GIF animation shows that you chose Select > Transform Selection before clicking the Flip Horizontal button. That means what got transformed is not any image pixels, but only the selection marquee. The evidence is you can see that the selection marquee flipped over, but the wing didn’t. If your intention is to transform the actual wing pixels, then after selecting, instead choose Edit > Free Transform (not Transform Selection) and then click the Flip Horizontal button.

In short:

Select > Transform Selection means transform the selection marquee, not the pixels marked by it.

Edit > Free Transform or any command on the Edit > Transform submenu means transform the pixels marked by the selection, which is what I think you want.

You have a "compound" selection. Invert the selection and try again.

Any update on why this is happening or if this will be fixed?

I added a gif animation. As far as I could tell I did the process correctly. No locked layers. I toggled the lock and unlock button just to be sure.

I checked and no the layer was not locked. My aim was to make a wing template by downloading a photo of a bee wing, scale it to size, print it, and then use it for polymer clay as a template.

Is your layer locked? Could you share a screenshot or video of what you're seeing? I'll move this back to discussion until we can confirm if it's a bug. ^CM