Per the discussion, Lightroom's behavior is to write OffsetTime in DNG files based on the local time zone of the PC. As a workaround, I've used exiftool to overwrite the DNG file's OffsetTime with the correct timezone that the photo was taken in. however, when I "update metadata" in lightroom, it ignores the offsettime that I set in the DNG file and writes it back to the PC's time zone, so I have to do it all over again.
If lightroom doesn't have a well defined way to set time zones, can it at least assume that the OffsetTime in the DNG file is correct and not change it? Then fixing OffsetTime can be a one-time manual thing. Not sure if this was the feature request referenced above.