Voltages not what I expect when charging a capacitor with another
But here's still no answer. It's corrected now:

The practical tests that I suggested in the comments, mainly swapping the capacitors, showed that they have radically different capacitances. Get new capacitors which are less different, then you get something more like the expected result.

The expected result, when two equal capacitors are connected parallel and one of them had 0V inital voltage and another had 10V: Both have 5V and the total energy stored in the capacitors as static electric field energy is halved, 50% is dissipated as heat and electromagnetic radiation.