As the name suggests FromUnixTimeMs() expects the Unix time in milliseconds.
This results in a DateTime that is -1456687 days or 4000 years in the past which is unrepresentable by a .NET DateTime and why this fails:
var fromMilliseconds = TimeSpan.FromMilliseconds(-125857756800000);
return UnixEpochDateTimeUtc + fromMilliseconds;
We don’t have an extension method to convert from Unix Micro Seconds, you’d need to /1000 to calculate milli seconds and use FromUnixTimeMs() or wrap it in your own FromUnixTimeMicroSeconds() extension method that does it.