Apple released a separate macOS High Sierra 10.13.3 Update for iMac Pro update on Tuesday. The update addresses the same “issue that could cause Messages conversations to temporarily be listed out of order” patched in the main macOS High Sierra 10.13.3 update. It also includes a fix for Macs connecting to SMB servers.
You can download the update through the Mac App Store. Apple has a standalone installer as a 1.53GB download.