When I look at the Deutsche Bahn site, EC 52 now goes from Milano Centrale direct to Spiez, departing at 11:20 am, arriving Spiez at 1:53 pm. When you look at the notes, it says:
Days of operation: runs daily, not 17., 18., 24., 25. Jun, 21. Aug until 10. Sep 2023, 9. Dec
So it looks like it is not operating Aug 21 to Sept 10. That tells me there is maintenance on the line. Other routings show a replacement bus between Sesto Calende and Arona in Italy on the direct route.
If you are going to Interlaken/Lauterbrunnen, best route looks like either the 8:10 departure on EC 156 or the 10:10 am departure on EC 158 to Luzern, then change to the train to Interlaken Ost.