Q2RE PMove(in a Q2RTX fork) and CONTENTS_LADDER. about ericw-tools HOT 4 OPEN

If a leaf has the solid content bit set, we intentionally clear all other content bits in the leaf (but preserve them in the brush).

So e.g. if the map file contains ladder | solid, we'll write the bsp leaf as solid and the bsp brush as ladder | solid.

I'm guessing this is what you're seeing?

As for why this is done, we were copying the original Q2 tools; this is the specific place this happens in qbsp3:
https://github.com/id-Software/Quake-2-Tools/blob/master/bsp/qbsp3/brushbsp.c#L695

and also here, two neighbouring leafs that both have solid in them get turned into pure solid:
https://github.com/id-Software/Quake-2-Tools/blob/master/bsp/qbsp3/tree.c#L180

I don't remember if we ever really figured out a good explanation for why the original did it that way (it might have been just so more solid stuff can get merged into single larger leafs?).

from ericw-tools.