It’s odd because microG absolutely 100% works with SafetyNet; when done properly, that is.
I’m currently on the April 3, 2022 FP2 build (LOS 18.1, so Android 11) of microG’s LineageOS fork and I pass SafetyNet attestation out of the box, no changes required.
It might be helpful to reach out to the developers of microG, or at least to the maintainers of the LOS fork, and see what they’re doing to their images to have them pass SafetyNet attestation…