Answer
Check whether that field is already pre-populated and registered by the platform's own Default method before the workflow even runs — explicitly registering an already-registered field as a Process Enrichment variable causes a duplicate-registration/serialization error, so for fields like this, just set the value directly in the workflow without registering it again. Separately, when the field is backed by a fixed enumeration, the value set in the workflow needs to be the enumeration's actual internal name (like AtFinalPosting), not its human-readable description (like "At Final Posting") — using the description where the internal name is expected is a common cause of a default-value-on-enumeration attempt silently not taking effect.