1.8V total battery voltage?
That's 0.9V per cell, you can't expect any meter to work at that voltage. The low battery warning comes on at 2.5V and effectively means "replace the batteries before taking critical readings". That's the same with every meter. That fact that it still works so far under the low battery cutout voltage is testament to the good design margin.
Should it cut out completely? Yeah, ideally, but few meters on the market do that that I am aware of.