Very intriguing observation

I replaced my batteries for my SKY sometime in december. The unit worked for a while but then the wind sensor showed some problems. I took it down, did check the batteries but they all seemed ok (the open end voltage was ok). So I reinserted them and the unit worked again. but not for more than a few days.
The brightness sensor also has problems.
I ordered new batteries, to replace them again. (didn’t do that yet)
However, this is what makes it a really interesting problem. The wind and light sensors have problems, but the battery and rain sensor work just fine and report data as usual.
To make it a very, very weird problem, look at the graph below (a bit squashed so it fits better on the screen). There are not many measurements, but there are measurements EVERY day around 6pm, midnight and around 4.30am.
Please come up with some implausible explanation as of why this could possibly happen.
Remember this is on a SKY, and battery/rain is working normally.