Pentax DL - battery voltage mystery

I've had one of these since 2005.

It takes four AA cells, which is one of the reasons I chose it, having
been burnt with the Sony lithium battery scandal, etc.

I have always used four NIMH cells in it - these are nominally 1.2V
when charged. And it was fine, taking hundreds of shots on one charge.

Recently, it started to indicate 'flat battery' very early.

I sent it to Pentax. After a long time (admin cockups caused by stupid
people) and £70 I got it back. I had some trouble establi****ng whether
they actually found anything wrong - apparently there is no
communication available with the 'engineer' doing the work...

It works the same way as it did before however. On normal alkaline AA
cells it shows full charge. On fully charged NIMH it shows only a
partial charge.

What could have gone wrong?

Products which can use either 1.5V or 1.2V cells tend to be selectable
somewhere otherwise they cannot tell if the batteries are going flat.

I am somewhat concerned because immediately before I sent it to Pentax
I did use it with alkaline cells (1.5V) and they still went 'flat'
pretty fast.

I well remember the Olympus OM2SP scandal where the camera drained the
battery when OFF...