I had a similar issue. The problem most likely deals with virtual textures and there’s a few solutions. The easiest is to enable Virtual Textures support in project settings. However, there are times when you don’t want VT and just want to use regular. In the project settings, set the Auto Import size to something above 8k and disable VT on import to avoid importing VT materials(doesn’t work 100% of the time).
The biggest benefit I see of VT is to sample materials from other actors. For example a rock with the same material will have snow on it when placed on snow biomes and moss when placed on swamp biomes automatically. Someone correct me if I’m wrong, but I believe the benefit of Virtual Textures deals with pixel overdraw on partial visible meshes. With VT the GPU will only draw the visible portion of the material. However, I’ve heard that nanite does the same.
Alternatively you don’t have to work with VT, with materials like MegaScans they often have a VT material and a regular material. You would need to switch out the materials (may need to temporally disable VT support and re-import).
Other assets may rely on a Runtime Virtual Texture of the landscape. Usually, their material instances have a bUseVT option or something similar, if not it’s easy enough to implement.
Mac may not support VT, I could be wrong. As a work around, you can dual boot Windows on a Mac machine, the benefit being windows is a better suited tool for working with UE. Then you’ll have one machine with Windows and IOS and can swap out depending on the appropriate tool for the job.