It is not technically a bug, but more so of an unexpected behaviour based on the UI. Not sure if it even would be right to “fix” it.
If you filter entities that have decimals, it won’t match what it is showing in the UI.
Example:
Here the % Complete is 99.9999999…, therefore not 100%, and thus technically correct—but not according to the UI.
A nice workaround could be made when formulas can mix numbers and text when you can X of Y.
I’m not really impacted by it. In the case I encountered it I don’t need to see the % Complete, it’s only a utility field for filters, so I will simply compare Children.Count() - Children.Filter(Completed = true).Count() and != 0 