I appreciate the effort to modernize ITA Matrix so that it can be kept around (instead of killed off like other Google products in the past). I do hope these issues can be worked through and improved upon.

FWIW I'm running into an issue on the frontend that makes it unusable for me. I'd enter a single origin and destination, enable advanced options. When I fill in all the routing and extension code fields, and then pick something that isn't "This day only" for both dates, advanced options will auto close and wipe out all of my advanced input. I hope this is something that can be fixed because it is pretty difficult to use in the current state.