This is not unexpected if we think about the law of conservation of energy. Putting the cap with the high ESR in parallel must dissipate some additional power in its ESR, and that additional loss will show up as an increase in the ESR of the combination.

Can you explain that ? It think you are forgetting something. If the Current is 1 A, then all current goes in that cap through the ESR of the cap. If you take two caps, you still have 1A total current. But now divided over 2 caps. So less current per cap and less current through each ESR

The problem is you have DC behaviour and AC behaviour.

For DC the inrush current is dictated by the capacitance and limitted by the ESR ( the RC time) once the inrush current is passed and the voltage over the cap is max only the ESR dictates the ripple current dissipation. So both caps will supply current to the circuit ( buffer), only limmited by there stored energy and ESR.

A small capacitance with low ESR will be empty during a surge while the big one still is delivering current, but if the small one is empty the big one has to supply the extra. Total current draw stays the same, dissipation can alter for good or worse. If the big one has lower ESR the dissipation will become lower if the smal one is empty. Because all current now goes through a lower ESR.

But at AC ( what you measure) the current through a cap is dictated by the reactance and ESR. If the high ESR cap has a low reactance at that frequency, most current will flow through that cap because of the much lower reactance and a small part through the second cap with low esr and high reactance.

So the dissipation has to be calculated per frequency and per cap. And in this case will be higher if only one cap was used.( all current then goes through the high ESR)

If the low reactance cap has a low esr and the other a high esr and high reactance the dissipation will be lower as in the first example but total dissipation wil be less because only a small part of the current goes through the high esr cap.

If frequency goes up, reactance of both becomes so small, the ESR is going to dominate and in that case they behave as two parallel resistors.

In your plot you see this happen. The ESR dives down at the end.

Today I took 4 MKT caps parallel. The ESR of one single cap was much higher as the 4 parallel.

1 philips 10 % 250V MKT 2.2 uF, measures 0.371 Ohm at 1KHz and 0.078 Ohm at 100 KHz.

The 4 caps parallel including extra copper to connect them, so extra ESR :

At 1 KHz i measure 0.097 Ohm ( if you correct for the extra copper 1/4th of the ESR)

At 100 KHz i measure 0.020 Ohm, ( again about 1/4th of the ESR of one cap)

Edit: hope it now makes more sense.