lol it's this, I commented there https://mods.factorio.com/mod/Flow%20Control/discussion/689b2cf05faeb82d7fc686a7; apparently I tried to debug it a bit but didn't find the root cause, and it's very strange that it happend because I activated their rather innocent sounding option "New subgroup for pipe items"... :-D
So I ended up using another mod, https://mods.factorio.com/mod/flow-config (as someone suggested in the thread I linked) this was my "solution". Of course, I don't know if that's the same issue as OP's or whether this "solution" works for them.
It was very strange; if you activated this mod's settings, when placing a ghost pipe, everything seems just dandy. Then, after waiting a bit, you wonder why no bots place the pipe. You hover over the ghost and see "pipe", no issues, looks fine. You alt-left-click on it, both in Factoriopedia and Recipe Book the correct non-hot variant is selected. Gorgeous.
But then you see the alert blinking on the lower right, "entities missing materials for construction" - and there's the hot variant icon. Same for pipe to ground. And then, they're never placed, no matter what you do (or I did).
Maybe that helps - maybe it's some sort of weird sorting issue (which might lead to improving the code of Hot metals, though apparently that's rather an edge case), maybe somehow the hot variant gets sorted before the normal variant because of whatever Flow Control does, but again, all's fine, the ghost looks fine - only the alert reveils that the bots are waiting for the hot variant...
