Seems to be a rounding error. A new machine filled up will use all soft tissue, output 20 refined eitr, and have >1 sap left (that is, a decimal value) that displays as 1. If you fill the machine again it will look like it has 20 sap but it's really 19.whatever sap. Once the machine starts again it will almost immediately go to 19 sap and you can put the 20th in.