It actually charges up another 0.4V.
The method used by Victron inverters to feed excess DC-coupled PV into the grid is to instruct the solar chargers to charge 0.4V higher. The Multi then detects this overvoltage and pulls it down by feeding that power into the grid. So the solar chargers attempts to push it up, and the multi tries to pull it down.
You may wonder why I bring up the feeding in of excess DC PV? Well, simply because KeepBatteriesCharged uses the same mechanism. It also instructs the solar chargers to charge 0.4V higher, and then the Multi pulls it down, but only up to a limit (which corresponds with your loads).
There is no control loop. That is the beauty of this setup. Control loops can get unstable, so if you can get away with not having one, so much the better! The 0.4V is deemed low enough that it will never be a problem, so if you have more PV than loads (when not feeding in excess PV and ess mode = KeepBatteriesCharged), then it simply rises a little higher than usual. Which is what you observe.
It is nothing to be concerned about. 52.8V is safe for your Pylontech batteries. In fact the battery wants 53.2V, but as you can imagine, if we used that voltage and added another 0.4V on top of it (53.6V) and then maybe another 100mV calibration difference, 200mV voltage drop over a cable, etc etc… you get way too close to 54V where the battery disconnects from the DC bus. Which is one reason that 52.4V is used as a base voltage.