Wrong filenames - time not matching

I deployed 40 AudioMoths in a forest for two months. However, when I now checked the SD-cards and the filenames, there is obviously something wrong with the filenames. I have files, that are named with as they were recorded within the night e.g. 11 pm - but I hear many songbirds singing on the file. Additionally, the date of the last recordings does not match the date of collecting the devices from the field. I put them out at the beginning of May and collected them at the end of June, but the last files are named with 20200515. However on those "youngest" files I can hear myself opening the enclosures and turning of the devices in my office. Does someone have a clue what has happened here to the timecodes of the files? Did the decives stop recording one day due to lack of energy (batteries lifetime should have run out before collection) but then started again to record when temperature rose again etc.? Or did something went wrong with the clock?

I configured them to record the whole day with sleeping for 1500 seconds and recording for 300 seconds with a sampling rate of 32khz.

Any hints concerning that problem would be great! Thanks! David Singer