when looking upwards from below the pipe sometimes just disappears

From two different angles (sorry that it's night-timed)

yeah for me it did; some pics will follow once I'm in the corresponding situation

But it seems to only happen if the quarry is not running

as fas i can see it's some small range of vertical angles at which it is not rendered properly the angle is somewhat dowards as you can see (in any upwards angles i haven't been able to reproduce it)
Oh and i might should mention i'm using optifine :/ going to test w/o it

For the pump ingame facing vertical value shows -31.3 to -39
and for the quarry it's 39 to 31.3 aswell (if being on the same level as shown above; if ascending(flying) the range decreases and shifts towards higher values)

optifine is probably causing it, remove it and see if you can still reproduce

Which version of BC is this? Because I noticed the same in BC 8.

Does this happens with the quarry too?

7.2.5 and the values were w/o optifine so yeah i can reproduce

I now tested w/ and w/o Optifine and in both cases the results were equal

Thanks. I'll look into it then.

alright must be a rendering bounding box issue then

Closed via commit, 7.2.6 should be released shortly.