Answer
This is an inherent limitation of the customization mechanism rather than something misconfigured — whenever an item is created by the system itself as a side effect of another process, IFS's system-generated creation path calls the specific API function responsible for creating that item directly, bypassing the additional UI-layer logic that a manual, user-driven creation in the standard screen normally goes through, and it's specifically that extra UI-layer logic that applies a custom field's configured default value. Since system-generated creation doesn't route through that UI layer, the configured default value never gets applied for a system-generated Shop Order. The practical workaround is triggering a Custom Event on Shop Order creation specifically to populate that custom field's intended default value programmatically, rather than relying on the field's own configured 'default value' setting to apply automatically for this particular system-generated creation path.