We are working on making multiple alertnate chains available in Staging in preparation for the multiple alternate chains that will be available in Production. The first step in this was generating and issuing from a new Staging Hierarchy that better resembles Production and has several chain paths. The Staging API should now serve a default chain of EE <-- R3 <-- X1 <-- DST3 (expired) and alternate chain of EE <-- R3 <-- X1 (Our first deploy of the new hierarchy had an incorrect default chain which we fixed to the above).
It's possible you are running into this problem:
You can fetch the STAGING Doctored Durian Root CA X3 from http://stg-dst3.i.lencr.org/