it is possible to see through walls with some resolutions

It seems that different graphical qualities does not show the hole, though, according to @superdupont-fr

[illwieckz]

It may also be related to “level of details” of patches.

[slipher]

I was able to reproduce this by setting the "medium" preset.

The crack occurs between two patches:

Each screenshot shows the runtime-generated collision geometry for a single patch.

The renderer has code for patches about stitching together edges and making sure levels of detail don't cause discontinuities, but this stitching stuff only applies within a single patch. So it seems this is an asset bug. The whole sewer should have been created as a single patch (assuming that doesn't violate some size limit), to ensure the rendering is continuous.

[slipher]

this stitching stuff only applies within a single patch.

I was wrong there. In fact R_StitchAllPatches tries to stitch together every pair of grids in the map.

BUT patch stitching seems to have no effect at all on the spot which this issue concerns. That the patches line up when r_subdivisions is 2 or 20, but not 8, seems to be a matter of luck. No stitching was done.