Audiomoths stopped recording early - battery issue?

Hello, we deployed AMs with 64GB cards (formatted to FAT32 with a Mac) and 3 x eneloop 1900mAh AA batteries. Calculated they should last ~8-9 weeks at our recording schedule. Have just found that the first 8 we retrieved all ceased recording within a day of each other, 4 weeks after deployment. The cards only have 19GB of data each, so probably not a card issue. Individual testing of AA batteries pre- and post-deployment showed average voltage of 1.34 each prior, and now each AM had at least one battery with voltage as low as 0.06-0.21V (but others up to 1.14V). Stumped, as a colleague had the same set- up with a slightly longer daily recording schedule and longer deployment, with no issues.

Am I right in thinking:

1) there is some critical minimum threshold in battery voltage, below which the AMs won’t work, despite our calculations they should have lasted twice as long?

2) if we decrease the amount of data collected per day they would last longer, and

3) does ticking or unticking the ‘enable low battery cut-off’ box in the configuration app make any difference to this?

4) is there a large error around the mAh drawdown prediction on the app that could explain our predicted vs actual data collection period?