I've already submitted this to MSFT via Feedback and resolved my issue for now, so this is basically informational for anybody coming across the same thing and searching for a resolution.
After installing the Win10-1803 GPO Templates, I'm presented with the below error:
Resource '$(string.Win7Only)' referenced in attribute displayName could not be found. File \\SysVol\...\Policies\PolicyDefinitions\SearchOCR.admx, line 12, column 69
I searched the folder on my PC where the files were installed. There's no SearchOCR.admx file in the new download, but there is an ADML file. After reinstalling the old and new ADML files, I found that the old file has a line for Win7Only, where the new one doesn't.
After reverting to the Win10-1511 SearchOCR template files, it's working normally again.