Voltage jumping around

I had lot of flights with my hexacopter that uses Pixhawk. I never had any major problems.
Lately I noticed that batteries voltage drops very noticeably in flight (from 24V to 16V) and then quickly recovers back to normal. Tried 3 different batteries, same problem. It doesn’t matter if I give a throttle, it happens in hover without reason.
Last flight my multirotor started to land automatically but I quickly switched to stabilize mode and landed safely.
Here is what my log looks like:

Any idea what is wrong?

I’ve gotten to where I quit using the MP/Pixhawk voltage sensor as under load it seems to ALWAYS under report. I have a Taranis radio with telemetry and have a lipo sensor and the numbers are always WAY off. I’m not sure how many others have this problem, but I’ve gone through three different power modules with that same outcome (the stock module, an Attopilot module, and now an Andoer 6S module…they all do it).