Canon ISO (gain) value is truncated to '0' - randomly

Using my Canon 6D (modified/cooled).

As seen below, the ISO (gain) value of an image is truncated to '0'. This is random.

Dark_2.00s_Oiso_0005.cr2 - BAD
Dark_2.00s_800iso_0000.cr2 - GOOD

It has happened with exposures of 30s, 10s, and 2s.

Out of 50 exposures of 2s each, this happened 5 times. There was a 4s delay between exposures.

Out of 50 exposures of 10s each, this happened 15 times with no delay between exposures.

Out of 22 exposures of 30s each, this happened 4 times with no delay between exposures.

It also happened when ISO was set to 1600 - truncated to '0'. Totally random.

Prior to today, my previous images were taken in September of 2021 with no problem like this. I was using the current Beta at the time.

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