Can't sort by metafield

I have a custom Algolia implementation that relies heavily on metafields. This is for a custom page that shows collections (not shopify collections) by designers, with pagination. They need to be ordered by collection and designer (meta.custom.designer_collection and meta.custom.designer) in order to group them. I can group the results using a custom hits widget, but I need to be sure that they are at least coming back from Algolia ordered by meta.custom.designer_collection. However, I can only add custom.meta.designer on the custom ranking and sorting page for some reason. I have 4 metafields in total (meta.custom.designer, meta.custom.designer_collection, meta.custom.category and meta.custom.subcategory), and I can’t understand for the life of me why only meta.custom.designer is choosable as a ranking attribute, while I need meta.custom.designer_collection.