I found some info on https://github.com/microg/GmsCore/issues/2697:
microG 0.3.6 got updated to check for the signature spoofing to be compatible with the latest version of Play Services. If you’re using an old version of signature spoofing, it will now say that signature spoofing does not work correctly in Self-Check. There is no immediate issue through this: If you have apps that use microG and you don’t update them, they will continue to work. However you should be aware that any updates to apps might cause them to no longer consider microG a valid implementation of Play Services and will show errors and/or ask you to update Play Services.