Unable to set UP board I2C clock speed to 100khz

sdo91

I am trying to set the I2C speed on my UP board to 100khz, but it doesn't seem to be working.

I set up the board using these instructions: https://github.com/up-board/up-community/wiki/Ubuntu_18.04

I changed the I2C Speed setting in the BIOS under Advanced -> HAT Configuration, but the when I measure the clock speed using a logic analyzer I still see a frequency of ~385 khz.

I am using i2c-5 (physical pins 3/5).

I think I have the latest BIOS/kernel versions installed:
BIOS version: UPC1DM25
kernel version: 5.4.0-1-generic #0~upboard5-Ubuntu SMP Fri Jan 7 11:53:57 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

I also tried changing the I2C mode from ACPI to PCI, as suggested here, but I see no difference.

So, how can I change the I2C speed to 100khz?