I moved your post to a different category. Just seemed better than Site Feedback which is for requests to improve the website.
I can readily recreate what you describe. I can't say I have ever chased the AIA for Staging certs like this so I'll accept your word that it worked correctly before.
Having the Doctored Durian Root CA X3 show up anywhere seems wrong.
For others, I recreated it by issuing a new staging cert today. Its leaf AIA points to the expected intermediate (Pseudo Plum E5 in my case).
Looking up that E5 intermediate shows the AIA as you describe. And, the E5 intermediate shows Issued by: Issuer: C=US, O=(STAGING) Internet Security Research Group, CN=(STAGING) Pretend Pear X1 which seems normal.
Retrieving the E5 AIA gets the below. I realize this repeats what you say I just think the Doctored Durian issuer is worth adding to the thread
Certificate:
Data:
Version: 3 (0x2)
Serial Number:
ed:5d:5b:c9:6d:fb:df:4d:3e:cd:6a:49:8d:d1:b3:c7
Signature Algorithm: sha256WithRSAEncryption
Issuer: C = US, O = (STAGING) Internet Security Research Group, CN = (STAGING) Doctored Durian Root CA X3
Validity
Not Before: Jan 20 19:14:03 2021 GMT
Not After : Sep 30 18:14:03 2024 GMT
Subject: C = US, O = (STAGING) Internet Security Research Group, CN = (STAGING) Pretend Pear X1