Comes down to a RAW interpretation. There are several ways to read the rules:
1) Specific items like DW Buckler, Elven Chainmail, are specific items and can't be enhanced.
2) Only magic special items (Celestial Armor) can't be advanced. Mundane special items (DW Buckler) can be magically enhanced.
3) Even special ones can be advanced like Celestial Armor to increase the AC bonus.
Mathias reads the rules as #1, you are reading it as #2 or #3. You will need to make a patch to fix it by replace-id the DW Buckler or some other way to fix it
Emphasis is mine, because I'm trying to get Mathias' attention here.
Paizo has inadvertantly ruled on this debate between these various rule readings with precedent.
In Serpents Skull #37 Chapter One (Souls for Smuggler's Shiv), the author, James Jacobs (Creative Director), lists a
+1 darkwood buckler as a treasure reward on page 21.
I'm not trying to be snotty, but I take that as some evidence on which rule interpretation is correct.
As it stands, I hate things in *red* too, because that signifies an error, but I read the thread. I have to use the editor I guess, but I'm wishing that wasn't the case.
EDIT: Alright, I just fixed that with the Editor in 3 minutes or less and it was comically easy, so I am embarrassed. HOWEVER, I actually use HeroLabs with my players along side a VIrtual TableTop (MapTools). In between sessions we reconcile PC .por files as character sheets and e-mail them back and forth to each other. That means sharing .user files too, and making sure those are current. Not that big of a deal, but I just wish it was uniform because that it is consistent with the rules, rather than hacking a user file get rid of the *red*.