I opened this issue on EnderIO thinking it was caused there, however I believe it comes from the newest update which changed some fluid things. "Added support for item stacks with fluid capabilities " With this newest version (tested on one before, works fine) I can't add any EnderIO fluids to fluid banks, inventory panels, killer joe etc.

Can't really do much more testing to be honest, can't think of anything else this could be.

Just writing to say I also have this issue and how no idea how to fix it apart from going back a few versions.

Did you read the answer in SleepyTrousers/EnderIO-1.5-1.12#3566 (comment) ?

@CrazyPants Any idea here? I could look at this earliest Monday or Tuesday.

How are you trying to add them? I need more detail than this.
Are you using conduits? Push/pull from a tank? Bucket in your hand?

This is fixed in ender core build 44

Sorry, was asleep during these comments. Ok so I've tested this, and for the most part it works. So I can now place nutrient distillation, cloud seed, fire water etc into fluid tanks via right click, and can now place bucket-filled fluids into the tank to empty them (via left hand side of gui). However, I can't seem to place the normal minecraft bucket in the right hand side of the fluid tank gui to take out fluid from the tank into the bucket. This doesn't even work when trying to take water/lava from inside the tank. Furthermore, trying to right click an empty bucket onto the tank to take out the fluid doesn't fill the bucket and just empties the tank, leaving nothing.

So basically to sum it up. I can give the fluid tank fluids with both ways (via right click and gui), however can't take out fluids (via right click with empty bucket and gui)

Also, I can't directly right click on Killer Joe to add nutrient distillation via bucket and right click, and same with the inventory panel however I can't remember if you could do that before.

Fluid conduits seem to work as intended.. Tested by using both Fluid tank and Vat containing nutrient distillation into an inventory panel.

Update: Tested this on new client, with latest forge/enderIO/enderCore, Still persists. Furthermore I found out that you can right click to retrieve fluid from the tank, however this only works in survival. In creative no fluid is added to the bucket and it stays empty. Not sure if that's normal or not. This is the same when right clicking a tank containing fluid with an empty one in hand, to switch the fluid to the one thats empty in your hand. Works in survival but not in creative.

Update 2: The disappearing fluid "bug" still occurs with the versions before the fluid change with Ender Core, so I assume this was always the case and might be a vanilla thing. However, adding nutrient distillation to Killer Joe and inventory panels via right clicking with buckets worked. Adding the bucket to the gui right hand side of a Fluid Tank to retrieve fluid which has already been placed in there also worked. This leads me to believe there's still an issue with the Ender Core update which changed some fluid stuff, since these were tested on the same forge version, with no other mods installed, only changes to Ender IO and Ender Core versions. (had to change IO due to Ender Core changes obviously)

Update 3: Reservoir also doesn't work on the newest versions. (can't right click with bucket filled with water, won't go in)

Thanks so much for the detailed reports IBeCrayZ
I will have a fix out soon. Just FYI this is all down to forge changing the way it handles fluids, and us attempting to support both the old and new systems. In adding support for the new, I broke some of the old.

'Furthermore, trying to right click an empty bucket onto the tank to take out the fluid doesn't fill the bucket and just empties the tank, leaving nothing'
I am guessing you where in creative mode, where this is the expected behavior.

I have just uploaded build 45.
Hopefully this should resolve the remaining issues.