I couldn't find a bug report for this behavior, though it may be related to the thread below on Hydrogen Gas, which issue gitlab lists as resolved.

https://mods.factorio.com/mod/planet-crucible/discussion/69fc0d0f43ad197e43997835

I'm sharing to find out if there's something about steam pipe behavior on Crucible that I'm missing or that perhaps there's some undocumented incompatibility with another mod.

Pipes carrying steam seem to be refusing to mix with other steam pipes carrying a lower-temperature steam and are producing fluid mixing alerts and not flowing; flushing seems to do nothing, too, since the lower-temperature pipe is empty.

I first noticed this when trying to set up hydrogen gas production with Electromagnetic Plants. I should point out that all of the steam my build is producing is from heat exchangers so it's all at 500 degrees. It seems that the input pipes to the EMP defaults to steam at 15 degrees, and for some reason isn't mixing.

https://i.imgur.com/es7sTSo.jpeg
https://i.imgur.com/73B36bY.jpeg

Now the behavior is appearing in pipe segments providing fuel to Pressure Forges.

https://i.imgur.com/xFtxVHa.jpeg

I did a quick check of my installed mods for something related to pipes but couldn't find one related to fluids or heat but maybe someone knows something that I don't.

https://i.imgur.com/fXkbeXE.jpeg
https://i.imgur.com/2CSbTGw.jpeg

This is a known bug in Factorio 2.1, and AIUI will be fixed before 2.1 is marked as stable.

In the meantime, the easiest way to work around this is to separate problematic segments with pumps. Entities of the same type can generally be grouped together, so at most you should only need to isolate pressure forges as one group, EM plants as another, and steam turbines as a third.

Wonderful!

Thanks a mil!

:)