So this odd behavior was caught during a unit test we were implementing.
In .NET Core 3.x, if a near 0 negative number is rounded and then converted to a string, the string will display "-0" instead of 0.
Framework and .NET Core 2.x don't exhibit this behavior.
double d = -.1;
Console.WriteLine(Math.Round(d,0));
Is this a new bug within Core, or some strange intended change?
This was a breaking change in .NET Core 3.0, for consistency between languages and IEEE-754 compliance.
Quoting from Floating-Point Parsing and Formatting improvements in .NET Core 3.0:
ToString(), ToString("G"), and ToString("R") will now return the shortest roundtrippable string.
The emphasis in the above is on "roundtrippable". Before this change, the negative zero "-0" did not roundtrip correctly, which was fixed by the combination of Fixing up the Double/Single parsing code to be correct #20707 and Roundtrip string format for single/double doesn't roundtrip -0 #9883.