I've restored the old GNU grep from a backup to a new file and it has the same issues in this OS. Even though I can now use -P with it.

If instead of searching through the Preferences folder instead look through my own app's folder where I also have a lot of PLIST files, then it matches on all of those without an issue.

If I tae one of those files that doesn't come up, load it into a text editor and re-save it, then it will come up in grep's match. I am not changing the encoding or anything else. File when re-saved doesn't matter if it's UTF-8, UTF-8 with BOM, etc. As soon as I cause the system to write to the file, then it will no longer show up in grep for the pattern containing the braces.

Again, WTF.