The Swiss website is buggy … using my iPad to book multi-city and full flex is impossible, if I don’t use their default flights but start go into each flight and change it, always to the last flight where it froze. But yesterday I decided to use my laptop (chrome) - initially, same results, but after I think 3 tries, it worked and go it ticketed with the exact date, time and flight I wanted. The LH site workaround works consistently.