Depth buffer getting cleared even when set to Don't Clear

I'm rendering portals, and I have two cameras that render specific parts of the scene, both into the same render texture. The first camera renders most of the scene, and the second one renders a few specific objects. The reason for two cameras is that for some objects I need to set a different near clipping plane.

The problem is that no matter what I do, I can't seem to get the second camera to stop clearing the depth buffer of the render texture. The first camera is set to the regular Skybox clear flag and the second camera is set to Don't clear, yet in the frame debugger I still see Clear (Z+stencil), which of course results in the second camera being "always on top" of the first.

What's going on?

Screenshots here: https://imgur.com/a/eaMSi7X

EDIT: I'm on 2019.2.6f1 if that's relevant.

 

After testing in a simplified project I believe it's a bug, so I've submitted a bug report for it. Sigh.