RAK7431 WARNING:DROP LORAWAN REQUEST

Hello, we have 12 x RAK7431s deployed with 38 more still to be deployed. These RAK7431s connect to a RAK7240 Gateway and a private instance of The Things Stack where they are configured like so:

Every 300 seconds each RAK7431 polls 2 x registers on 2 x modbus devices - a total of 4 x polling jobs.

Our problem is that a RAK7431 will work fine for many hours, sending regular uplinks. But then suddenly it will stop sending uplinks. After many more hours, it will start sending uplinks again. This happens randomly at different times for all of our RAK7431s.

Today I connected via serial to an RAK7431 which had stopped sending uplinks and I saw these log messages every 300 seconds when it was running polling jobs:

[13:07:17:499] SYSLOG:4:DTU Tx : 01039C9B00029BB4␍␊
[13:07:17:859] SYSLOG:4:DTU Rx : 01030444992000272C␍␊
[13:07:17:878] SYSLOG:2:WARNING:DROP LORAWAN REQUEST␍␊
[13:07:17:878] SYSLOG:4:DTU Tx : 01039D3200010A69␍␊
[13:07:18:123] SYSLOG:4:DTU Rx : 0103022710A278␍␊
[13:07:18:142] SYSLOG:2:WARNING:DROP LORAWAN REQUEST␍␊
[13:07:18:142] SYSLOG:4:DTU Tx : 20030028000242B2␍␊
[13:07:18:178] SYSLOG:4:DTU Rx : 200304000184ACF98C␍␊
[13:07:18:198] SYSLOG:2:WARNING:DROP LORAWAN REQUEST␍␊
[13:07:18:198] SYSLOG:4:DTU Tx : 2003002A0002E372␍␊
[13:07:18:234] SYSLOG:4:DTU Rx : 200304000362CB53C6␍␊
[13:07:18:249] SYSLOG:2:WARNING:DROP LORAWAN REQUEST␍␊

[13:12:18:364] SYSLOG:4:DTU Tx : 01039C9B00029BB4␍␊
[13:12:18:755] SYSLOG:4:DTU Rx : 01030444A50000FEE0␍␊
[13:12:18:774] SYSLOG:2:WARNING:DROP LORAWAN REQUEST␍␊
[13:12:18:774] SYSLOG:4:DTU Tx : 01039D3200010A69␍␊
[13:12:19:014] SYSLOG:4:DTU Rx : 0103022710A278␍␊
[13:12:19:033] SYSLOG:2:WARNING:DROP LORAWAN REQUEST␍␊
[13:12:19:033] SYSLOG:4:DTU Tx : 20030028000242B2␍␊
[13:12:19:069] SYSLOG:4:DTU Rx : 200304000184ACF98C␍␊
[13:12:19:089] SYSLOG:2:WARNING:DROP LORAWAN REQUEST␍␊
[13:12:19:089] SYSLOG:4:DTU Tx : 2003002A0002E372␍␊
[13:12:19:124] SYSLOG:4:DTU Rx : 200304000362D7520F␍␊
[13:12:19:138] SYSLOG:2:WARNING:DROP LORAWAN REQUEST␍␊
...

I tried using AT+SYSLOGLVL=6 to increase the log level but there was no change in output.

After issuing AT+RESTART the RAK7431 joined the network and immediately started behaving as expected:

[13:30:21:599] EVENT:6:RESTART␍␊
[13:30:22:803] EVENT:0:STARTUP␍␊
[13:30:22:843] SYSLOG:4:OTAA Join Request␍␊
[13:30:29:288] EVENT:7:OTAA_FAILED␍␊
[13:30:34:332] SYSLOG:4:OTAA Join Request␍␊
[13:30:39:393] SYSLOG:4:OTAA Join Success␍␊
[13:30:39:393] EVENT:1:JOIN_NETWORK␍␊
[13:30:39:434] SYSLOG:4:LoRa Tx : ␍␊
[13:30:39:498] SYSLOG:4:DTU Tx : 01039C9B00029BB4␍␊
[13:30:39:896] SYSLOG:4:DTU Rx : 01030444504000DF12␍␊
[13:30:45:078] SYSLOG:4:LoRa Tx : 810001000A0101030444504000DF12␍␊
[13:30:50:344] SYSLOG:4:DTU Tx : 01039D3200010A69␍␊
[13:30:50:588] SYSLOG:4:DTU Rx : 0103022710A278␍␊
[13:30:50:612] SYSLOG:4:LoRa Tx : 8100020008020103022710A278␍␊
[13:30:55:858] SYSLOG:4:DTU Tx : 20030028000242B2␍␊
[13:30:55:902] SYSLOG:4:DTU Rx : 200304000184ACF98C␍␊
[13:30:55:939] SYSLOG:4:LoRa Tx : 810003000A11200304000184ACF98C␍␊
[13:31:01:185] SYSLOG:4:DTU Tx : 2003002A0002E372␍␊
[13:31:01:227] SYSLOG:4:DTU Rx : 200304000363081207␍␊
[13:31:01:264] SYSLOG:4:LoRa Tx : 810004000A12200304000363081207␍␊

Can anyone advise as to what :WARNING:DROP LORAWAN REQUEST is indicating and what might be causing the RAK7431s to raise this warning for hours before going back to normal?

Our configuration is:

NPSG_DEFAULTS = {
    "CLASS": "C",
    "JOINMODE": "OTAA",
    "REGION": "AU915",
    "PUBLIC": 1,
    "ADR": 0,
    "DATARATE": 3,
    "BAUDRATE": 9600,
    "CHANNEL": "5:5",
    "DATABIT": 8,
    "STOPBIT": 1,
    "PARITY": "EVEN",
    "DTUMODE": "MODBUS",
    "MODBUSTIMEOUT": 3000,
    "TRANSPARENT": 0,
    "CONFIRM": 1,
    "RETRY": 3,
    "ENABLEPOLL": 1,
    "POLLPERIOD": 300,
}

Device firmware is 1.1.0063_Release

Thanks!