Final image suddenly getting masked after flattening

[winterludes]

I just did a bunch of resizing on this image and had no issues.  But today when I flatten it and resize it, the final image is getting masked it seems, but that shape does not match any of the masks on my asjustment layers.  I have no idea why this is happening all of a sudden.  I've tried flattening it in various ways and restarting my computer.  This is what the final image looks like.  It looks fine in photoshop, but in apple preview it looks like this, and same with Topaz AI.  

It's like there's some phantom mask suddenly stuck in the image.  Like I said, I flattened, resized, sharpened, and exported this image in multiple sizes just a couple days ago without this happening.  

Can anyone explain what is going on?  Thank you!

[winterludes]

Actually I just went back and checked my previous exports of this image and it looks the same in preview!  So it's not something new, there's something to do with this image.  It does look fine in photoshop so maybe it will print fine from photoshop, but I need to be sure and also figure out what on earth is happening.  

thanks!

[winterludes]

It looks like I figured it out.  i had somehow an extra color channel that was a mask exactliy this shape, and I erased it.  I have no idea where it came from, but erasing it seemed to have no affect on the image.  Maybe i mistakenly created a mask there instead of on one of the adjustment layers.  Here's what I found.  Can anyone explain what may have happened and if just deleting this extra channel is the right way to go?  thanks again.  

[D Fosse]

That's an alpha channel, which can be used for saving selections (among other things).

Lots of applications interpret an alpha channel in TIFF as transparency and therefore masked out. That's probably what happens here. It's not strictly according to spec, but has probably become a widespread convention. Windows does it natively, and apparently MacOS does it too.