Practical cryptographic systems rely on a true random number generator (TRNG), which is a necessary component in any hardware Root-of-Trust (RoT). Hardware trust anchors are also integrated into larger chips, for instance as hard-IP cores in FPGAs, where the remaining FPGA fabric is freely programmable. To provide security guarantees, proper operation of the TRNG is critical. By that, adversaries are interested to tamper with the ability of TRNGs to produce unpredictable random numbers. In this paper, we show that an FPGA on-chip attack can reduce the true randomness of a TRNG integrated as a hard-IP module in the FPGA. This module is considered to be an immutable security module, compliant with NIST SP 800193 Platform Firmware Resilience Guidelines (PFR), which is a well known guideline for system resilience, and it is also certified by the Cryptographic Algorithm Validation Program (CAVP). By performing an on-chip voltage drop-based fault attack with user-programmable FPGA logic, the random numbers produced by the IP core fail NIST SP 800-22 and BSI AIS31 tests, meaning they are not truly random anymore. By that, this paper shows that new attack vectors can break even verified IP cores, since on-chip attacks are usually not considered in the threat model, which can still affect highly integrated systems.