If you're shooting raw images LR uses the full "raw data" image area to create the HDR image. You will need to crop it manually to 16:9.
No, just the opposite. When I use raw images, do an HDR Merge, the "merged" .dng image appears "squashed" in height, re the composite images in the Merge. .
Further, see "Lightroom CC problems with HDR Merge"
The best place to report a "known" problem:
Click on 'Report a problem' at the above site.
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