Zero byte files with 1/1/1970

I recently purchased an Audiomoth 1.1.0 and am using a SanDisk Extreme 32GB. I have done 4 trials overnight and all went well until the fourth. I left the Audiomoth out for 6 nights with sample rate of 192kHz. It recorded for 2 nights (up to about midnight on the second night) with no problems, then for the next 3 nights, the date changed to 1/1/1970 10am and the files had zero bytes (although the file names still recorded the date correctly). There was no data for the 6th night. The total file size was 19GB, so not full. When I retrieved it, two batteries were still good, one low - I use endeloop rechargeable batteries. The SD card was formatted as MS-DOS (FAT) - I deleted previous files but did not reformat the card for each trial. Can anyone suggest what the problem might be? Thanks.