I have noticed that if you try to sell bonds any clicks you make on the + or - part of sell bonds will increment the buy bonds counter instead, meaning you can only sell 20,000 units at a time.
This appears to only happen on the latest version which is currently the 9.1.24 release. I mention this due to the fact that the past few versions apparently resolved some bugs in bonds but it appears that it may have created a new one.