There has to be a logical reason for this..
There is. Orientation metadata in source file is not compatible with Lightroom.
PS - the orientation changes for the same reason other things change after importing - Lr first shows camera-created jpeg, then re-rendering based on metadata interpretation and default settings...
So, to solve the problem, consider:
* checking camera vendor for upgraded firmware.
* if you used pre-import software, report problem to software vendor.
* if you did not use pre-import software, then report problem to camera manufacturer.
* if camera manufacturer insists the problem is Lightroom, then report problem to Adobe here:
In the mean time, the solution probably involves exiftool, probably a command like:
"/path/to/exiftool" -orientation="TBD" "/path/to/my/file"
where TBD stands for to-be-determined
exiftool is readily findable for download from Phil Harvey using internet search.
When you determine the proper command, please post it here. If you can't determine the command, post a link to a sample file, and maybe somebody will help..
PS - consider checking whether Lr5 has the problem too. Also, in cases such as this, it's good to specify make & model of camera, and operating system etc.