NOTE: I'm on 1.11.2 and on a single player world.

I believe it is a bug, not sure.

P.S. Items and liquids are still transferring through them and I'm on buildcraft version 7.99.6

this is purely a visual thing, something in those chunks (or neighboring that got send to the client at the same time) is throwing errors when syncing to the client, this is causing forge to choke on the packet and the pipes are not receiving any info from the server about what they actually are

you're gona need to check your logs for what threw the inital error, most likely pams harvestcraft if you have that installed

Ok

missed this earlier: duplicate of #3685