In ME1 the free_shipping:true flag may exist in the data, but Mercado Libre does not automatically interpret it as "free shipping" on the front-end or apply ME2's automatic benefits. Free shipping visibility and benefits are designed for ME2; for the front-end to display free shipping with automatic benefits, the item must be in me2 mode.

If the account or user preferences have ME2-prioritized coexistence enabled (e.g. tags such as me2_available or commercial preferences), the platform may prevent the conversion to ME1 and return not_specified. The reason is usually account configuration and commercial preferences; converting to ME1 may require commercial intervention (KAM) to adjust shipping preferences.

Yes: many automatic display rules and benefits (e.g. "Free shipping" with benefits) apply preferentially to items in ME2 mode and also depend on the price threshold and other internal policies. The global benefit cannot be forced via API if these conditions are not met.

No. Nationwide free shipping activation depends on automatic rules (price, logistics type such as ME2) and cannot be forced via API without meeting those requirements.