I have not tested it but 2.7V at VBAT is pushing it due to the nature of transient current spikes due to RF transmit, etc. By default the nPM1300 feeds the nRF9151 with 2.7V for IO and uses VSYS for direct use for the RF circuitry, etc. When running from battery through the nPM1300, brownout will occur around 2.7V as well.
