Try content aware fill
Your images are taken with different focus points? It looks as though it's selected data from a photo where the background was in focus and the foreground flowers were not.
There is some noise in your data (high ISO?) that may have influenced Photoshop's decisions.
On a hunch, to test the theory that the noise is a factor, try downsampling all the images to 50% in each dimension, saving the files as PSDs, then running PhotoMerge on those PSDs. If it selects from the exposures more appropriately then you'll know.
If the noise really is the factor, maybe you could do some things to influence it, such as cropping the images ahead of time so that only the in-focus parts are available to PhotoMerge.
ISO 100, f-stop /9.0, ss 1/10, using a tripod. The images were 24 mp, but the final image was reduced to 450 x 405 mp and the jpeg reduced to level 4 so I could get it onto the site. I was a little surprised when I saw it. I thought Photomerge was smarter than that, the reason for this post. Since then, I re-examined the original images and found out my image was out-of-focus in that part (not the focus of the image), and Photomerge was that dumb. I guess I have to make my images in focus all the way--I won't be able to trust Photomerge to know anything about the focus.
Thanks for your help Noel.
Thanks helping me think out of the box, but I tried it and it didn't help.