Hi @korw,
Yes, I have run into this over the years. Not the CPU cores applet in particular but various ones. It's like a time race where the applet that fails starts before the panel is fully ready. I always figured the second login is successful because things are cached and the timing changes.
Here's a technique that has worked for me but I rely on my personal startup file:
When those applets fail, open a terminal and test this:
mate-panel --replace &
This should verify the panel starts up ok.
Then, the permanent solution is place this exact command in the startup file after some delay like 2 or 5 seconds, whatever works. You will likely see a bad panel replaced during boot with this in place.
There are ways to make a single startup entry with a delay but I've used this personal startup method for so long I'd have to research and test the sh commands.