Why can't I find some customer accounts in search on Shopify?

Already did this, no help was offered and no solution.

Not being able to search for our customers when creating an order create a much larger workload for us later on.

You cannot add a customer to an already processed / fulfilled order and the new “Edit” button on orders is basically useless, you cannot attach a customer to an order using that either.

This was not an issue a week ago.