From July 26 to July 30 traffic on the high speed line will be suspended because of planned works in Florence on the Ponte al Pino bridge: https://i.postimg.cc/sf6sTJgc/Immagine-2026-03-23-225945.jpg
Florence is halfway between Rome and Venice/Milan.
That's the reason you can get tickets of trains running before the 26th and after the 30th. The warning on trenitalia.com reads "temporarily not available", it has nothing to do with the timetable change that will happen on June 14.
Just wait, in the worst case you'll have to take a Trenitalia substitute bus between Florence Campo di Marte and Florence Rifredi stations. They did the same in January and no tourists were harmed during the trip. But they could re-route some trains going north on the slow line and you will avoid the bus trip.
ps No union would dream of calling a strike 4 months in advance. Who would ever join?