This isn't a bug in so much as it's an inconsistency on delta.com.
When doing a revenue ticket search the equipment will be displayed as "767" if you're on a BE-configured 767-300ER on a dometic route...e.g. SEA-ATL 1135am departure.
However, if you search for an award ticket that same flight will show equipment as "767-300ER". Why does it show correctly on the award ticket search but not a revenue ticket?
Note: the domestic configured 767-300 appears as such on both revenue and award search results.
-RM