As anyone using Shield encryption knows, there are all sorts of considerations documented here:

• General Shield Platform Encryption Considerations
• Considerations for Using Deterministic Encryption
• Use Encrypted Data in Formulas

None of these are relevant

Upon detailed investigation, the issue appeared to be pod-related

• PROD: failed (na81)
• Staging: works fine (cs62)
• Dev sandbox: failed (cs43)

That is, the identical formula field referencing identical cross-object encrypted (probabilistic) fields rendered NULL in two pods but not a third.

The workaround was surprising:

• Create a new custom formula field on Case called My_Identifier_Clone__c. Copy-paste the exact same formula as in Case.My_Identifier__c.
• Change the references to the formula field in the Email Template to be {!Case.My_Identifier_Clone__c}
• Retest - email renders correctly

I suspect I could also have deleted the original formula field Case.My_Identifier__c and add back but this would be problematic to deploy through a Devops pipeline without two steps

As anyone using Shield encryption knows, there are all sorts of considerations documented here:

• General Shield Platform Encryption Considerations
• Considerations for Using Deterministic Encryption
• Use Encrypted Data in Formulas

None of these are relevant

The workaround was surprising:

• Create a new custom formula field on Case called My_Identifier_Clone__c. Copy-paste the exact same formula as in Case.My_Identifier__c.
• Change the references to the formula field in the Email Template to be {!Case.My_Identifier_Clone__c}
• Retest - email renders correctly

I suspect I could also have deleted the original formula field Case.My_Identifier__c and add back but this would be problematic to deploy through a Devops pipeline without two steps