Because it is "working as intended" even if that intention is not what should have been. It's a poor definition of a bug imo. If there was a limit of 10 items and they sorted correctly then this would be a design limitation and adding more would be a feature. If there is no limit, but after a certain amount they stop sorting properly then that is a bug. A software bug is an error, flaw, failure, or fault
in a computer program or system
that causes it to produce an incorrect or unexpected result, or to behave in unintended ways. Most bugs arise from mistakes and errors made by people in either a program's source code
or its design
It may not be desirable for roll20 to fix these less common cases, but it is indeed a bug by that definition imo.