Hi, Andrew.
I’ve reproduced your issue on my computer. It’s an error of Photo merge/HDR. See the screenshot as below.
The resolution must be 7296 x 3648. Yours is too small.
And I’d like to highly recommned you to try on #SingleDNG. It’s faster and more comfortable, IMO it has enough image quality. How do you think?
#SingleDNG developed in LR Classic and THETA Stitcher.
Good luck!!
Toyo
PS.
If you want to do the bracketing and develop of Z1’s images, you should use PTGui pro. It’s perfect.