When HEIC photos from an iPhone are imported, they immediately have Metadata Status: Changed On Disk. Editing the photos changes the status to Conflict Detected.The incorrect status prevents metadata changes from being written to the .xmp sidecars when the option Automatically Write Changes Into XMP is enabled.
To reproduce:
1. Download and unzip these sample photos:
https://www.dropbox.com/s/h9xwwlk11d3jdcc/heic-metadata-bug-2019-06-13.zip?dl=02. Import the photos into LR using the import option Add.
3. Wait a few seconds and notice the metadata status changes to Changed On Disk:

Tested on LR 8.3.1 / Mac OS 10.14.5 / Iphone 7 / Ios 12.1.4