Merge lane priority advances the lane with the highest priority to the top of the release queue, even when it's not the next in line logically speaking. You can find the lane priorities on "Intellimerges" screen

The lane with lowest number priority has the highest priority. If all lanes have the same number, they all have the same priority, therefore, they release based on their slug readiness and their place in the release queue.

You can also find the lane priority details on debugger: