I'm experimenting with a 3-way sync between Capture One, Photo Mechanic Plus, and Lightroom Classic now, and PMP updates metadata from either sidecar or the original DNG when using Update IPTC/XMP command from Tools.
What I'd like to find out is whether it could detect those files that have either changed as DNGs or their sidecars have changed and update metadata only for those. I can bastardize that discovery via sorting via Modified Time, reverse, abd kind of eyeball the range of recently rated/color-classified images from other apps, but would rather have them dectected automatically. In C1 that's achieved by setting Auto Sync XMP to Load, and in LrC by doing Synchronize Folder and checking Scan for metadata updates. The latter only picks up the DNG changes from PMP though, not XMP sidecars appearing from C1.