TL:DR MPPT converts power from the panel at a higher voltage to an almost equal amount of power at the battery at a lower voltage. Victron says there must be at least 2V difference for the MPPT to provide the best performance. In the individual panel case, you are very close to this 2V limit so the MPPT output will be less than expected. In the parallel case, there is more input power from the panels but you are still close to the 2V limit. In the series case, you have much more than 2V difference from PV to battery so the MPPT can work more effectively.
More details:
Looking at the RNG-100DB-H 100W panel I-V curve I included above (similar to your panels but not the exact curve since I couldn't find it) you will notice that the panels output the same current near 5.75A for a wide range of voltage. The panel output power is the voltage * amps. If you operate the panel at less than V(max power) or Vmp, you will lose output power. For example, the same panel with the same amount of sun with the output directly connected to a battery at 14.4V * 5.75A = 82.8W. You lose 100W - 82.8W = 17.2W. You can see that by looking at the green line with the power in Watts on axis on right.
The MPPT circuit adjusts the panel output voltage to find the place where its output power is maximum, the peak in the green line. For this panel in full sun, that will happen when Vmp is around 18.9v * 5.29A = 100W. With MPPT, the panel output voltage can vary while the battery stays at 14.4V. The power to the battery will be close to the power from the module (minus some small loss to operate the MPPT), but since the voltage is different, the current must also be different. From the panel, 100W / 18.9V = 5.29A. To the battery, 100W / 14.4V = 6.9A.
Check out this paper from Victron for more details:
https://www.victronenergy.com/blog/...ar-modules-to-the-new-mppt-charge-regulators/
Since the Voc of your two panels in series is around 40V, well below the MPPT maximum input of 100V, it would be much better to connect them in series. With more intense sun, the panel output will increase so you might get the full output with them in parallel. Series will give better results on cloudy (or smoky) days. Parallel might make sense if one of the panels is shaded or they face opposite directions.
Without the 250W load you added your almost fully charged battery would not draw much power from the panels. Adding the 250W load is a good way to take that factor out of the equation.