I experience this. When I scan my images to tif and then edit in lightroom, the embedded xmp data for the current state of the image is reflected when I view the previews with Adobe Bridge or PS. However, the embedded preview according to my Mac OS does not reflect any of this and creates a confusing situation when sifting through files using the OS natively. However, converting to dng does not allow previews to be displayed by the Mac OS and reminds me I have to use Bridge.
I also want to convert TIFF to DNG. I scan my 4x5 pinhole negatives to TIFF and load them into Lightroom for processing and storage. The camera produces considerable stretching in the corners of the image. I want to create a lens profile (for a lens less camera) to take care of the distortion. It is my understanding, which is often imperfect and growing more so as time marches on, that profiles will only work against DNGs. Also, you can validate the checksum of a DNG which will warn you of bit rot so you can refresh the active file from backup.
I thought I read so where that whatever file format you give Lightroom it processes it as a DNG. If that is the case then 1) you should be able to apply a profile to any file you can open in Lighteoom, and 2) you should be able to export the open file as a DNG.