I was able to fix my issue with cross-environment access. For those who are interested the issue had nothing to do with AX AIF; it rather had to do with SIDs of the accounts being out of sync. In other words while the login/domain/password for the account have appeared the same between the environments, the SIDs turned to be different. From what I understand WCF RSA-encrypts messages with SID being part of the key hash. This explains why it has worked for a little while until lately - as turns out someone has dropped the service account from the AD in the target environment and recreated it from scratch. As soon as we matched the SIDs between the environments the issue was no longer.
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