I do use a Bronica wit a 135 panorama back, similar to Xpan. Lightroom totally sucks most of the time when I try to stitch them, often it refuses to stitch completely - sometimes it adds strange distortions to the final image.
So it would be perfect to get special stitching options regarding scanned images - positives and negatives even as Dng´s.
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Are you using the Perspective option when stitching. I think the others (Spherical and Cylindrical) would lead to the distortions you mentioned.
Yes I did. LR stitching option is often a mess when dealing with filmborders, it counts the whole picture in even when cropped. If I export the files to PS via “edit in pano mode” it works like a charm, but then I end up with Tiffs… So a cropping function wich is not easily distracted by scanned pics and end up in a RAW file would be heavens
I do the same with scanned images from my Xpan. This is done in two passes, one scan aligned to the left side of the image, one to the right side so there is an overlap of about 7mm in the middle of the image. I use a dedicated panorama software (in my case Autopano Pro, which is unfortunately not available any more) to stitch these images, which usually works pretty well in the automatic mode.
Sometimes I have to adjust the horizon or some of the control points, but this is rare. I often edit these “Panos” afterwards in LR, although they are “only” 16 bit Tifs.
I don’t see the point of adding a stitching mode, as this is a very special field with special problems and as you have seen “one does it all” software can be limitated in their usability.
Have you tried using a panorama software like Microsoft ICE (https://www.microsoft.com/en-us/research/product/computational-photography-applications/image-composite-editor/) or something like PTGui (there is a trial mode available)?
The point is to keep everything in raw. No big files, better editable. It is not only for xpans but also for medium format shots which i do stitch often out of 2 or 4 shots. Here works LR better but still has its limitations or distortions. ps works miles better, but again I loose the Raw abilities. 
I see the point,
but so far I have never gotten good results from the different panorama programs available when using raw files. I usually got better results when I preprocessed the images in LR and then either used tif or even jpg files for rendering the pano (not only for the special case like here).
Thanks for the request, @MattiMalandro - I completely understand what you are talking about and it’s something we’ve discussed on the Negative Lab Pro Users facebook group.
I also find that Lightroom adds strange distortion to my DSLR-scanned panos most of the time (even using “perspective” stitching mode). I believe that Lightroom is still expecting that this is a typical series of panoramic shots, and not series of flat scans that just need to be stitched. The two of these require different types of stitching algorithms. It’s also more difficult, I believe, to calculate the stitching on the DNG files as you have added layers of complexity dealing with with the embedded lens correction profiles.
For me, it wouldn’t be that difficult to add a stitching algorithm that would be optimized for flat scans, BUT, I don’t believe I’d be able to keep it fully RAW the way that Lightroom currently does. Which may defeat the point? I guess it would be quicker than exporting and doing in photoshop, but would lose the benefits of the raw workflow.
So in my mind, I think the ideal solution would be able to document cases where LR is misbehaving and open a ticket with Adobe to see if they can fix it internally. I’ve found Adobe to be pretty good addressing issues that I’ve run into in the past.
Also, I will say that it helps for scans to be PERFECTLY flat and parallel to each other. For instance, here’s an Xpan 2-shot stitch I just did with my new setup.
If you look at the border, it came out close to perfect in Lightroom. But the input was really really flat and two shots were perfectly inline using MK1 Film Carrier.
So it’s possible to get good results, but the margin of error is very slim in Lightroom.
Nate
Creator of Negative Lab Pro
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Nice catch 
… I recently got rid of all bars/connections in my 35mm Filmholder (still flat ), so I don´t have to move the pano (open filmholder, move, close…) each time I scan the second part - works good so far. I do also agree that LR needs a plain variant for stitching scans. Let´s open Tickets and pray
I wonder if anyone has found better solution on this topic. I found the pano merge in LR is really unstable. Sometimes I got the best results from perspective mode, but sometimes it’s from spherical mode.