Hi,
Well, within the groups, still the most relevant matches will be the first – but the groups may not be ordered by relevance.
You can try the following configuration:
– Increase the relevance value of the custom field matches, and decrease everything else: https://i.imgur.com/sPMbgX8.png
– And try also this option: https://i.imgur.com/7ldOQYH.png
With that, the custom field (including _sku) relevance will be much higher in contrast to other fields – so each result within each group should be the best matching by relevance. The second option should theoretically reorder the groups to show the ones with the most relevant matches first.
There is still no guarantee the groups will go exactly as you want them, because a relative weighted relevance is used to set the group positions.