I have a storage bin that's underwater. It's set to take slime at priority 9, i.e. a lot.
It has 19837 kg of slime in it, so it's not full.
I'm watching a dupe try to put 7.6mg of slime in this storage bin, over and over. The dupe puts it in, and it pops out and goes into the water. This isn't some odd scheduled/priority problem because the dupe tries hard repeatedly.
I tried to work around by making another bin and playing with priorities, but it can't put the slime in this bin either.
I think I made the problem go away by emptying one of the bins, which created a giant slime wad that the dupes could cope with.
I have no idea how to reproduce this.