Despite LightRoom's long term weakness with timezone handling, I've always been able to store a full timestamp with tz info in the "Date Created" field. Recently (maybe since the LR 8.3 update), it has started stripping this out. This appears to only affect RAWs with XMP sidecars, and does not affect JPEGs. I think it happens when metadata is read from file.
I have a workflow where I save my metadata after import, process it in GeoSetter (I've been using it with LR since before the maps module was added, and frankly it's better for some things), and then re-import it. GeoSetter has always insisted on choosing a timezone before saving, which I suppose is to comply with exif standards/conventions,
I copied the my files at each stage of this process, and you can see how LR saved without any tz info, GeoSetter added, and after import back in to LR and save again, LR has stripped it:
This is a picture I imported and processed in May, with tz info for Melbourne: RackMultipart20190629106195sa9-9b3812ac-7167-46cb-9e1c-f2af8c1eb2e1-1121187482.png
This is a picture I recently imported and processed in June, missing the CEST+DST tz info for Germany: RackMultipart20190629620681wsp-25e1eab7-dd42-4646-9686-6201b6391bb7-404235796.png
At this point, if I manually add "+02:00 to the "Date Created" field in LR and then save, this shows up correctly in the XMP file. Therefore I think it's a regression in LR's metadata reading function. In this case, this what we get in the XML file (look at the value of "photoshop:DateCreated":