I collected 4 buckets of lava in one of my tanks and then tried inputting it directly (i.e. using a hose) into a Tinker's Construct seared tank. Didn't work as expected: it generated some "ghost lava" (oooo spoooooky), which couldn't smelt anything. The Tinker's tank appeared full, but the backpack one appeared full as well. Then, as I compacted the Smeltery into a Geochest and then opened it again, the ghastly lava was gone (nb: Geochests instead keep any inventory item or tank fluid or thingumajig contained in the blocks they grab). However spilling it on the ground and then collecting it with a bucket appears to work just fine, as does filling the bucket directly in the inventory. Today I'll try the same with water and see if the problem persists.

NOTE: I didn't try any other backpack than the cow one, as of the time of this writing.

Seems like a client-server sync issue. I'll check it out.