Not sure what machine.info() 'System free heap' field is telling me exactly but it is a third of its previous value: 1.15.0.b1 =390096, 1.13.0.b1 = 1161380.
I toggled between the 2 updates without changing my code.
Has the 1.15.0.b1 update really used up that much memory when compared to 1.13.0.b1?

(sysname='LoPy4', nodename='LoPy4', release='1.15.0.b1', version='v1.8.6-849-baa
8c33 on 2018-01-29', machine='LoPy4 with ESP32', lorawan='1.0.0', sigfox='1.0.1')
System memory info (in bytes)
MPTask stack water mark: 4848
ServersTask stack water mark: 976
LoRaTask stack water mark: 2004
SigfoxTask stack water mark: 2968
TimerTask stack water mark: 2012
IdleTask stack water mark: 248
System free heap: 390096

LoPy4, LoPy4, 1.13.0.b1, v1.8.6-849-3d556fb on 2018-01-10, LoPy4 with ESP32, 1.0.0
System memory info (in bytes)
MPTask stack water mark: 5556
ServersTask stack water mark: 976
LoRaTask stack water mark: 2028
SigfoxTask stack water mark: 3020
TimerTask stack water mark: 2052
IdleTask stack water mark: 568
System free heap: 1161380