I first saw it with Vishroom Stew from Automagy, I was attempting to use lava capsules. The Automagy dev saw a capsule incorrectly output Flux Goo, and another user in that topic also saw Vishroom Stew.

Today, I actually tried investigating more than I had the first time. I tried giving myself a bunch of different capsules so I could see what each one might theoretically put into a tank. The first one I used was "molten shiny", which put "molten shiny" into the tank -- but then so did everything else.

I thought maybe it might be world-specific, so I made another world and tried the same experiment. Everything output molten shiny into a tank.

I then thought, maybe it was per instance-of-you-loading-the-client, so I restarted my client. Still molten shiny! It's also not "the first capsule you use in the world". After the first time, I made sure to NOT use molten shiny first, and even doing lava and non-TiCon fluids still put in molten shiny.

SECONDLY: When you right click on Tinker's Construct tanks (seared tank, seared window, seared glass) it puts 1000mB into the tank, and 1000mB out in the world as a fluid block, which proceeds to flow everywhere. I asked #tinkersconstruct if this was even remotely possibly on their end, and mDiyo commented:

[21:08:55] @mDiyo Tinkers' Construct uses standard liquid mechanics. If anything, it's on the Witching Gadgets end.

Will be fixed in the next version. Forge's FluidContainerRegistry is stupid and doesn't like NBT data.