I noticed an issue in 2.0.151 that I've traced down to the change in underscore handling 33090c0. This issue did not exist on 2.0.143.

There seems to be some issue with the logic that chooses what property to map to.

If you have a class like this, the setter on DisplayName never gets called and AltName never gets set.
This only seems to happen if the property has a backing field, if that backing field is above the other field (Display name in this case), and if the backing field has a { get; set; }.

private string _displayName { get; set; }
public string DisplayName {
    get => _displayName;
    set {
        _displayName = value;
        AltName = _displayName;
    }
}

public string AltName { get; set; }

If you move the backing field below DisplayName, the setter on DisplayName gets called as normal. It also works fine if you remove the { get; set; } from the backing field.
So this works fine with the backing field below.

public string DisplayName {
    get => _displayName;
    set {
        _displayName = value;
        AltName = _displayName;
    }
}

private string _displayName { get; set; }
public string AltName { get; set; }

The query I'm doing is just something like, using Npgsql.

return conn.QueryAsync<MyClass>("SELECT display_name FROM some_table;");

I also have DefaultTypeMap.MatchNamesWithUnderscores set to true.