I taught a class last week using InfraWorks 2024, Model Builder. The project is located in Switzerland, around 15 students accessed the site via Model Builder the same time. They all received flat terrain, some students tried it again during the lecture break. They receive projects with terrain. I think I had this issue before but I thought it was solved.
We used a workaround and imported swisstopo raster terrain data, which is for free.
BTW: In Forma I can access the swisstopo data too. InfraWorks should have the same.
Peter