Almost always because a paid method was left uncapped. Any method with an upper limit of "No limit" still qualifies on orders above your free-shipping threshold - and Shopify can pre-select that paid rate instead of Free Shipping.
How to fix it:
Open the Shipping Rate Group and check every paid method's cart-value (or weight) range.
Give each paid method an upper cap just below your threshold - e.g. for free shipping at $75, cap paid methods at $0–$74.99.
Make sure only Free Shipping covers the range above the threshold ($75–No limit at $0).
Sync to Shopify and re-test with a cart over the threshold.
💡 Thresholds are currency-specific - if you sell in multiple currencies, set the threshold for each.
