Cura 4.4 missing object parts
Using the preview feature as you have is a good method to determine that the original model is flawed. It's not unusual to discover detailed models have reversed normals or non-contiguous surfaces, which will not print.

In a simple example, consider a cube for which five of the six faces are properly described by the design software. This would result in all eight vertices being present, allowing the software to present the "missing" face, but the slicer is unable to create this aspect. The cube is no longer solid and is presented as seven zero-thickness panels, which can't be printed.

It is necessary to repair the broken models, by closing gaps and reverting/recalculating normals, but that requires skill in other software.

Some slicers will attempt to repair damaged models, but some are so severe, the results you've experienced are the result.

If you want a confirmation of a failed model, load it into free Meshmixer, run Analysis/Inspector and expect a "pincushion" of highlighted failures.