Issue with sleep mode in low voltage situation

Hey Team,
We have a device using PXP_reporter project.
When we put the device in extended sleep mode, it advertises for 8 seconds then stops if the voltage is less than 2.90V.
If the voltage is over 2.90V we don't have a problem and the device maintains advertising and operates fine.
While not in sleep mode the device operates fine right down to 2.17V which is our curcuits minimum voltage.

We have tried numerous tests including turning off brown out detection, various different power sources and even different hardware designs, all with the same result of some form of crash or hang-point of advertising while in sleep mode when the voltage drops below 2.90V.

When we try booting a device straight into sleep mode, we see it boot, there is roughly 8 seconds advertising time and then the device stops advertising and gets stuck. When we try to use an interrupt to wake it out of sleep mode, advertising doesn't come back on.

Another experiment we did:
1. Powering system with 2.5V: system advertise and then go to low power mode. At low power mode I measure V14 (this is Radio power, 1.4V) I saw 0.3V
2. Powering system with 3.3V: system advertise and then go to low power mode. At low power mode I measure V14, I saw 1.3V.

Would you have any suggestions on what might be causing this issue?

Best regards,
Ben