As I discovered later, the reason for triggering the memory leak bug was a hardware issue. I describe it here to help people in the future having the same problem:
I was powering the boron via the VUSB pin with a powerful (3A) supply (always stable 5V here, ensured with scope), but I didn't increase the PMIC input current limit. The default is 500mA which is not enough for the Boron 2G/3Gs current peaks. More details: Correct power supply for Boron 2G/3G without LiPo battery
This lead to voltage drops on VSYS which caused the modem to be unresponsive. The DeviceOS then restarted the modem, which leaked memory, also observed in thread linked above and confirmed here:
As this occurs infrequently, depending on cellular signal conditions, it is hard to debug. To reproduce this issue, I connected an electronic load to VSYS and pulled current (~650mA) until the voltage dropped to 3V for about 300ms. This causes the modem to become unresponsive but is enough to let the Boron not restart. Logs (level WARN) look like this:
0000063889 [gsm0710muxer] ERROR: The other end has not replied to keep alives (TESTs) 5 times, considering muxed connection dead
0000070480 [app] INFO: network_status_connecting
0000070629 [comm.protocol] ERROR: Event loop error 3
0000070632 [system] WARN: Communication loop error, closing cloud socket
0000083129 [app] INFO: network_status_connected
0000083137 [system] ERROR: Failed to load session data from persistent storage