Answer
Add an IFS Projection Read task before the Gateway/condition check, and read the actual current record explicitly, rather than relying on whatever value the workflow's own trigger context initially handed you. Checking the field's value from that fresh Read (instead of trusting the triggering event's passed-in value directly in the Gateway condition) resolved this exact cross-triggering symptom — the trigger-supplied value isn't always a reliable representation of the record's actual current state for a field-level validation check.