Depth Buffer inconsistancy?

I have an issue in which 3 different DX9 games, all running on different engines, all produce the same issue using ReShade and MEReborn. The depth buffer disappears, and reappears, seemingly randomly. I've disabled all Nvidia forced effects to no avail. The depthbuffer will work fine, then vanish, then come back, for no consistent amount of time or in situations that have any real connection. It can be gone for 5 seconds or indefinitely. Any clue as to what could cause this?

Yes, the detection algorithm runs every 30 frames, to change the source in case the game switched to a different depth buffer when loading into another scene/area for instance or to disable access when multiplayer traffic is detected. Now this can have an effect like you experienced in some cases, with different reasons: Game may have done some extensive online communication, drawing order changed drastically (like from looking at the floor to looking at the sky), and ReShade thus finds another depth buffer more appropiate, ...

It does happen mostly when looking around. But these are basic games with mostly static scenery. This issue was not encountered in ENB, in any of them. And it doesn't switch to a more appropriate depth buffer, it disables it entirely.

ReShade is an application, it has a different understanding of "more appropiate" than the user: It can only follow an algorithm and the result of that one does not necessarily match the expected output.
Depth access being disabled entirely does mostly happen due to the game producing some unusual network traffic.