I came across a strange behavior and would like some clarification what’s happening here.
I did create a simple immutable struct

public readonly struct MatchingPair : IEquatable<MatchingPair>
{
    private MatchingPair(Assets asset1, Assets asset2, bool validate = true)
    {
        if (validate)
        {
            InvalidMatchingPairException.ThrowIfEqualOrUnknown(asset1, asset2);
        }

        if (asset1 < asset2)
        {
            Asset1 = asset1;
            Asset2 = asset2;
        }
        else
        {
            Asset1 = asset2;
            Asset2 = asset1;
        }
    }

    public static MatchingPair Create(Assets asset1, Assets asset2)
    {
        return new MatchingPair(asset1, asset2);
    }

    internal static MatchingPair InternalCreate(Assets asset1, Assets asset2)
    {
        return new MatchingPair(asset1, asset2, false);
    }

    public Assets Asset1 { get; }
    public Assets Asset2 { get; }

    public override string ToString()
    {
        return $"{Asset1}<->{Asset2}";
    }

    public bool Equals(MatchingPair other)
    {
        return Asset1 == other.Asset1 && Asset2 == other.Asset2;
    }

    public override bool Equals(object? obj)
    {
        return obj is MatchingPair other && Equals(other);
    }

    public override int GetHashCode()
    {
        return HashCode.Combine((int)Asset1, (int)Asset2);
    }

    public static bool operator ==(MatchingPair left, MatchingPair right)
    {
        return left.Equals(right);
    }

    public static bool operator !=(MatchingPair left, MatchingPair right)
    {
        return !left.Equals(right);
    }
}

Assets is a simple enum nothing fancy.
After that I created an Exception class that will be raised when the Assets are invalid.
Lazy as I am I did not write every testcase instead I used the DynamicDataAttribute from the MSTestv2 Unit Test Framework.
However this does not work as expected because MatchingPair is losing it’s values.
The minimal Test looks like

[TestMethod]
[DynamicData(nameof(MatchingEngineExceptionConstructor4Data))]
public void MatchingEngineException_Constructor4_Test(string? message, MatchingPair? matchingPair, string expectedMessage)
{
    if (matchingPair.HasValue)
    {
        Assert.AreEqual(Assets.Btc, matchingPair.Value.Asset1);
        Assert.AreEqual(Assets.Etc, matchingPair.Value.Asset2); // Fails unexpectedly
    }

    // ...actual test...
}

To create the test data I did the following

private static IEnumerable<object?[]> MatchingEngineExceptionConstructor4Data
{
    get
    {
        foreach (var message in new[] { null, "Céẞßö." })
        {
            var expectedMessage = message ?? "Error in the matching engine.";
            foreach (MatchingPair? matchingPair in new MatchingPair?[] { null, MatchingPair.Create(Assets.Btc, Assets.Etc) })
            {
                if (matchingPair != null) { expectedMessage += $" (MatchingPair '{matchingPair}')"; }

                yield return [message, matchingPair, expectedMessage];
            }
        }
    }
}

However, in the Unittests both Asset Properties are set to Assets.Btc which can only mean that they are set to the default value (0). But I would have expected them to hold the actual values I did set. The values are set correctly the expectedMessage I put together is proof of it since it shows the correct values.

Can someone bring some light to what is happening here.
I guess a possible fix is to convert the struct into a class, but I want to avoid that.