0x8048d9 was logged twice last night while parked and locked, with nothing going on

So please verify there really was nothing going on. Maybe you didn't see the vehicle was actually active due to reduced log levels or missing log entries in the leaf code?

Logging to SD card was globally at info level, except with debug for can, esp32can, mcp2515 & events. Here's all I saw:

2025-01-26 19:56:25.997 GMT I (92702057) housekeeping: 2025-01-26 19:56:25 GMT (RAM: 8b=79720-82596 32b=12560 SPI=3263128-3285676)
2025-01-26 20:01:25.997 GMT I (93002057) housekeeping: 2025-01-26 20:01:25 GMT (RAM: 8b=79720-82596 32b=12560 SPI=3263128-3285644)
2025-01-26 20:03:57.337 GMT E (93153387) can: can1: intr=4800273 rxpkt=4799065 txpkt=900 errflags=0x8048d9 rxerr=0 txerr=128 rxinval=0 rxovr=5 txovr=0 txdelay=51 txfail=281 wdgreset=0 errreset=0
2025-01-26 20:03:58.337 GMT E (93154387) can: can1: intr=4800319 rxpkt=4799065 txpkt=901 errflags=0x8048d9 rxerr=0 txerr=135 rxinval=0 rxovr=5 txovr=0 txdelay=51 txfail=282 wdgreset=0 errreset=0
2025-01-26 20:06:05.337 GMT E (93281387) can: can1: intr=4800334 rxpkt=4799065 txpkt=902 errflags=0x8048d9 rxerr=0 txerr=134 rxinval=0 rxovr=5 txovr=0 txdelay=51 txfail=289 wdgreset=0 errreset=0
2025-01-26 20:06:26.007 GMT I (93302057) housekeeping: 2025-01-26 20:06:25 GMT (RAM: 8b=79720-82596 32b=12560 SPI=3263128-3285676)
2025-01-26 20:11:25.997 GMT I (93602057) housekeeping: 2025-01-26 20:11:25 GMT (RAM: 8b=79720-82596 32b=12560 SPI=3263128-3285612)

2025-01-26 23:16:26.017 GMT I (104702057) housekeeping: 2025-01-26 23:16:25 GMT (RAM: 8b=79720-82596 32b=12560 SPI=3263128-3285488)
2025-01-26 23:21:26.017 GMT I (105002057) housekeeping: 2025-01-26 23:21:25 GMT (RAM: 8b=79720-82596 32b=12560 SPI=3263128-3285456)
2025-01-26 23:23:04.347 GMT E (105100387) can: can1: intr=4801533 rxpkt=4799065 txpkt=903 errflags=0x8048d9 rxerr=0 txerr=131 rxinval=0 rxovr=5 txovr=0 txdelay=51 txfail=888 wdgreset=0 errreset=0
2025-01-26 23:26:26.017 GMT I (105302057) housekeeping: 2025-01-26 23:26:25 GMT (RAM: 8b=79720-82596 32b=12560 SPI=3263128-3285488)
2025-01-26 23:31:26.017 GMT I (105602057) housekeeping: 2025-01-26 23:31:25 GMT (RAM: 8b=79720-82596 32b=12560 SPI=3263128-3285456)

The best option to capture anything going on with the bus over the night is activating an unfiltered can log to the SD card.

Do you mean setting the global log level to verbose?

The normal behaviour of a parked car is doing 12V battery maintenance every few hours, i.e. topping up the 12V battery from the main battery. That will normally also cause a limited CAN wakeup of the ECUs taking part in that process.

With the MQTT metrics I observe charging of 12V ceased at 18:40. The battery then went into gradual decline starting at 12.8V until it reached 12.4V the next morning.

Another option is some plugin or script doing a wakeup to query values.

The only activity I have is an MQTT idle update interval of 10 minutes. Overnight this just sends things like free memory, 12V voltage and signal strength.

As you don't see any issues yet, proceed with both buses active to see if the new timing solved that.

Both buses now set to active.