Firstly I apologise for the delayed response here.
In late 2019 we switched to a different search algorithm for invoices, the original method was not scalable and was causing exessive load on our database. We’ve now switched to a “full text search engine”, it is significantly quicker and allows deep searching of invoice lines from the quick search box in the left hand menu.
One of the trade offs here is that it won’t return partial word matches, so you’d need to enter the whole word. We are looking to introduce support for wildcards (e.g air*) in the future but we don’t have a fixed timescale for this just yet.