FPS drops when buildings are hidden behind terrain

FPS drops from about 60 to 30 when the camera points toward an industrial area hidden completely behind a mountain. When the same area moves outside the camera view, FPS returns to 60. It looks like buildings, conveyors, and other objects behind terrain are still being rendered or processed instead of being culled. Screenshots attached.

OS: Windows [10]
CPU: AMD Ryzen 7 8840HS
GPU: AMD Radeon 780M
RAM: 24 GB

I notice the Radeon 780M, that's a laptop, isn't it? Google Yup, lower-midrange mobile graphics... Oh dear, Ryzen 7 8840HS is a high-range mobile processor with onboard Radeon 780M, rather than a separate die, so system memory is probably dual-purposing as VRAM, and you're almost certainly graphics-bottlenecked. According to Google, that's a relatively modern high-range CPU paired with the equivalent of a GTX 1060 (mid-range, ten years ago). Fairly typical for "Integrated" graphics, but well within the advertised minimum system requirements (as far as performance goes, at least, though below the recommended specs).

Occlusion culling offers very little gain for the vast majority of gameplay, as it is rare that objects are completely occluded by other objects: it's pretty much only when you have a low camera angle like this.

Doing the calculations to compute whether things should be culled has a cost - both adding code complexity (and potential bugs) - and reducing frame rate as you have to calculate occlusion. You pay that cost whether or not things are actually culled, and in the significant majority of viewing angles nothing would get culled.