Upgrading to
<PackageReference Include="MSTest.TestAdapter" Version="4.1.0-preview.25514.12" /> <PackageReference Include="MSTest.TestFramework" Version="4.1.0-preview.25514.12" />
fixed the issue as @Youssef13 suggested above.

I also had to change from Sdk="MSTest.Sdk/x.y.z" to Sdk="Microsoft.NET.Sdk" since the first one seems to overwrite packages even if and update element is added to the project file:
<ItemGroup> <PackageReference Update="MSTest.TestAdapter" Version="4.1.0-preview.25514.12" /> </ItemGroup>
That behavior is a bit unpredictable but not related to this issue.