It appears to be routine processing. Just wait for the next step in the process.
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I am surprised that it has taken this long, and I would recommend that you schedule an Infopass appointment, which might trigger its issuance. However, it could simply be that the biometric department is particularly busy. However, there is no chance that the USCIS would take any action before the biometrics. As far as I can tell, no one really reviews the I751s before the biometrics appointment is issued.